As Cult its important to accumulate enough money in the early game and always have enough available for hiring hired swords that meet your demands. for if youre lucky and one of the hired sword heroes has the right mutations and no injuries. then he can have his shot to become part of your dream team.
That means you only buy the most important spells and skills, mostly on your Leader (mainly curse resist and master version of Weapons of Destruction until you reach the midgame)
Try to get to roughly 1000-1500 Gold in the Bank as soon as possible and keep your balance at least at that level at all times. Only buy blue/purple gear. and skills after that. Always having enough Gold left for hiring a promising Hired Swords with great mutation(s) is really the #1 priority.
I will describe in this guide further down, for what exact Mutations to look out for and why.
Paying more than 1000 gold for a 5+ hired sword may look like alot of gold. But note that, if you hire those you can also give them their skills for free. so they are really worth the investment if they have the right mutations.
Still, with not spending so much money on skills you will find yourself in an abundance of money very soon (unless you really luck out on the hired swords big time)
What you will lack alot more than gold is faction rank.
For the longterm its highly beneficial to send your Wyrdstone only to the third faction in the beginning until you got it to rank 4 (aside from shipment requests)
The reason for this is, you will get less Gold but you will need less influence each rank that way.
The other factions will offer you alot more gold for your Wyrdstone once your 3rd faction is past rank 2. still, only do that if its really urgent. if thats the case and you really need the gold for a hired sword, you can send the other 2 factions Wyrdstone too, but then try not to hit the next rank with those, because this will increase the reputation requirement with the third faction and delay your warpguards significantly.

I recommend going mostly Darksouls in the early game and only 1 or 2 Brethren. The Darksouls are just a good Meatshield for protecting your heroes. while their defensive capabilities arent that great (low weapon skill) they can do quite alot of damage and they do their job just fine. Its always better to sacrifice Darksouls than risking injuries on your Heroes. They are supposed to be cheap and expendable. and since your Henchmen will level alot faster than your heroes (since heroes will be dismissed if they turn out to be disappointments at their Level 1, 5 and 10 Milestones) you will have to dismiss your henchmen at some point aswell, or your total army strength gets too high for your fresh new heroes. This neccessity to protect your Heroes becomes especcially true after they hit Rank 5 and have already 2 desirable Mutations. You will avoid getting them injured or killed at all costs! Ill talk about the right Mutations in the later section.

With the Darksouls you want to dump your points into mostly defensive abilities. I build them as suboptimal parry tanks with decent dmg output, so points go into Toughness, Initiative and Weapon Skill. Alternatively you can go Agility and use just cloth armor with them and use dodge instead of parry, but i prefer light armor and parry with lots of hitpoints. but either way, both builds do their job just fine-ish. you want to use 2hand weapon until they hit level 3 and always charge when possible, after that weapon and shield (when they get their 4th offensive point) and walk into melee range before attacking. At level 8 you can swap to 2handed Weapon again and swing twice without charging.
Do not bother learning any skills on them, since they will get dismissed sooner or later. Just equip them as good as you can.

As for the Brethren, you put your points into Agility, Initiative and Ballistic Skill. I prefer the Bow over the Shortbow since they have a better range, but if you get a better quality Shortbow it works just fine aswell.
their secondary weapon is dual wield or 2hander until they hit level 3, and then give them a good Onehander like a Mace or Axe and a Shield.
The only skill you want to give them is quick reload. Its all they need until the lategame. Save your money.

Once you have the ability to recruit Warpguards, i recommend going at least 4/5 Warpguards, who will all become heavy armor tanks with great mobility.
I suggest to build them like this:

6/12 Strength
9/15 Toughness (req. for Resilient)
13/15 Agility (rest of the points, more melee resistance)
give them physical grimoire once you can to max out agility and one more toughness

3/12 Leadership
9/9 Intelligence (req for Knowledge: Mordheim)
10/12 Alertness (rest of the points)

17/14 Weapon Skill (ideal to save 3 points for when you get access to training: weapon skill book)
3/9 Ballistic Skill
5/12 Accuracy

Skills:
Web of Steel
Defensive Stance
Adrenaline Rush

Shield Specialist (swap with Awareness if you like to use Halberds often)
Armor Proficient
Knowledge: Mordheim
Resilient

Let me explain why i build them like this. There is alot of argument going on between the benefits of maxing agility or toughness for pure tanking. Its true, that if you max melee resist and go max agility and Awareness instead of Resilient your Warpguards will be realllly hard to hit. but their big weakness is if they get crit they are dead. melee resist aint helping if youre stunned. with this build, they might get hit slighty more often (59 Melee Resist compared to 66 with highest quality purple shield and melee resist rune; add +10 in defensive stance), but they also bring 19% crit resistance to the table compared to just 7%. this really makes them more reliable to hold the line. and thus this build is supposed to bring together the best of both worlds - having high agility and melee resist while getting some nice crit resistance along with a few more hitpoints.
I recommend using Mace on Warpguards and a Halberd in the second slot.
Defensive Stance is for situations in which you cant parry.
The reason for picking Adrenaline Rush is that this gives your Warpguards the ability to swap to Halberd if you want them to finish something off and dont need the tankyness anymore. Adrenaline Rush enables to switch Weapons and then still being able to swing twice with it at level 8+ and 5 offense points. After that, in the next round, you can even charge to the next foe and swing a second time if you use Adrenaline Rush again. I found the utility of this really huge because with the Cult missions usually dont last very long because of their amazing damage output. But dont go too overboard with it, because it costs you ~10% of your Health every time. (30 out of 295)
I usually only do this on those who havent tasted their own blood yet.

If you like you can keep one Darksoul with a 2handed Weapon like a 2handed Sword or Axe in his primary slot in your squad and focus him on Strength. Once they hit level 8 they can become great extra damage dealers with 2 attacks per turn if you dont charge. (or charge with adrenaline rush)
but usually you dont need this slighty extra damage because the real cheese comes from the Heroes and Impressive anyways. And this Warpguard build gives them the ability to hit quite hard too. Purple Halberds can be quite scary. And whats nice about Halberds is that they allow you to parry too.
I recommend 4 Warpguards and a Brethren in the lategame, once you have the ability to recruit them.

Let me say at last, im aware that the suggestion and recommendation to go for Warpguards with the Cult may seem shocking and strange for the role player type of fans of the Warhammer world. If you want to keep the cult true you can just ignore this and keep playing with slow lagsouls... xD
Im not really that kind of role player (actually im not even that much familiar with the warhammer fantasy world at all)
Im just a player who likes to look at the numbers and play around with them and optimize everything as good as i can. and that made the desicion easy for me. This guide isnt supposed to tell you what to do, its merely meant as a suggestion! :)

Just be warned, getting to Warpguards requires roughly 3000 Reputation if you go straight for the 3rd Faction and ignore the other 2 for the Reputation Bonus. (and as mentioned above, that means a bit less gold; but dont worry about that, because gold is infinite in the lategame looong before the factions are even close to maxed out)
Thats an equivalent of 200 Clusters. At Rank 10 you should get around 6-7 Clusters worth of Wyrdstone every mission on average. Thats around ~30 Missions in the lategame. Since in the early warband-levels you dont get as much ressources you cant expect to be able to get Warpguards before roughly 70 missions. So getting there isnt something you do in a single day, but its not thaat long either.
To me personally, aiming straight for Warpguards was always was a blessing in disguise that kept me going. Because that made not only the right Heroes with their Mutations hard to get, but there was this other thing i had going on the side where i could visibly make progress - even if i had setbacks with having to dismiss heroes, if the god of chaos decided to screw them. ;)

in the endgame you have 2 spots for Heroes in your Warband. You will want to get one Mutant Ranger and one Marauder melee guy for tanky damage. You will already have plenty of tankyness once you hit the Warpguard milestone.

There are desirable Mutations, acceptable Mutations and bad Mutations. In the following section im going to rate the greatness of the mutation for the Mutant Ranger in particular to the best of my knowledge and experience.

Foot:
Claw Foot.
Movement range +2 meters
Ambush and Charge range +2 meters
Chance to pass Climb, Leap, Jump Down tests +10%
10/10
Arguably the best Mutation in the game. And one of the main reasons why a promoted Brethren can never be as good as the Mutant when it comes to Ranged utility.
Mobility is king in this game and this single Mutations gives mobility worth 6 Skillpoints and a passive slot, along with other bonuses like higher success chance when climbing. Archers benefit from highground advantage, so this is really nice.
If you get this on your Mutant on Level 1 or on a Hired Sword Mutant you will want to level him up all the way. If you can get a 5+ Hired Sword Mutant with this and a non-arm Mutation its basically jackpot. then chances are more likely than not he will become part of the A-Team.

Body:
Thousand Eyes.
8/10. +10% ranged/melee resistance.
Its one of the better non-arm Mutations. He wont be your prime Frontliner, but with this he has the potential to do so, once he gets engaged on and you switch to 1h+Shield.

The Eye.
+10 Initiative
+10 crit Resistance.
7/10
More initative is nice for archers and being more resiliant to crits is just fine too.

Head:

Wyrdstone Horns
7/10
Wyrdstone Resistance +50%
On damage received, gain a warp buff that increases Melee and Ranged Resistance by 5% for 1 turn. Stackable.
good for looting Wyrdstone, making you a bit more tanky should your mutant get caught. + it looks really cool ;)

Extra Face.
Dodge and Parry chance +15%
6/10
While not as good as Thousand Eyes, it provides some extra defensiveness if your mutant should get engaged on.

Featureless Face.
Perception is free and increased by 20%.
5/10
While its kinda meh and you dont really need it at all, ill rate it that high for the Mutant because at least he didnt lose one of his arms. If you have this and something amazing like a Claw Foot at Level 5 its still worth to keep going.
*edit: i might undervalue this, i heard its really useful on the ranger.

Demon Soul
Immune to All Alone.
Magic Resistance +25%
7/10
One of the better Mutations and it looks quite cool too. Mutant is one of the few Cult units who isnt Immune to Fear and getting high Magic Resistance is also great.

Any Arm Mutation and you can dismiss him, since this means he henceforth cant use a Bow anymore.

I usually dont spend alot of gold on the skills on him before he hits 10. the only thing he needs to work is basic reload.

Suggested final stats:
Strength 4 / 12
Toughness 12 / 13
Agility 14 / 16

Leadership 2 / 10
Intelligence 9 / 12
Alertness 12 / 15

Weapon Skill 5 / 15
Ballistic Skill 15 / 15
Accuracy 15 / 15 (need tome of martial and accuracy for this, req for master headshot)

Suggested Skills:

Adrenaline Rush (M)
Headshot (M)

Quick Reload (M)
Knowledge: Mordheim
Sharpshooter
Trick Shooter

If you dont like Adrenaline Rush, alternatively i would recommend going for Master Handshot (-20% hit chance for target for 1 turn) to make good use of the odd 9th OP. This is a great skill in PVP aswell, especially if your frontline is stacking alot of Melee Resist, making it for the enemy almost impossible to hit your guys. 9 OP are enough for 1 Headshot, 1 Handshot and 1 Basic Shot, or 3 Basic Shots and 1 Handshot if you want just raw damage.
Suggested Equipment:

Cloth (ballistic skill rune), Helmet
Bow (best rune is 20% highground hit chance or the 12% Hit Chance rune in all situations)
secondary Mace+Shield

Purpose: Great ranged DPS, Stunning biggest Threats

Something i want to say from my personal experience:
If you get like a level 1-4 Mutant with something nice like Claw Foot from the Hired Swords, i would advise against using your skillpoints and martial statpoints on him yet.
I had one with Claw Foot who got the Crusher Mutation at Level 5, but he already learned alot of Ballistic Skill and Quick Reload already. He would have made an amazing Melee guy. I was like biting in my keyboard when i had to click the dismiss button.... XD
So if you can afford to do so, and your Warband is strong enough to carry a Mutant with unassigned (martial) statpoints for a while its something to consider ;)
After Level 5 its probably worth skilling him, odds to lose an Arm at this Point are maybe ~40%. Better than a coinflip. And from 5-10 it takes too long to let him keep being a burden for your team in my opinion.

I suggest Marauder as the melee guy because of his superior physical stats and higher weapon skill. you can use a dodge Mutant aswell, or heavy armor Mutant if thats what you want to go for.
Now there are great offhand Mutations which look nice on paper, but that also means his number of attacks are quite limited. if you use any special attacks 2, with adrenaline rush 3 are possible. Tiring effects arent that great on heroes, so the offhand Mutations are better on the Chaos Spawn.
You can also use another Mutant Ranger or promoted Brethren as the second Hero, if that fits your playstyle more. Even Possessed Builds work well, but for this purpose im going to talk about a debuff / dps / tank marauder:

Desired Mutations:

Crusher (right Hand Mutation)
Effect: Melee attacks gain the following benefits:
Critical hit chance +8% (with purple Armguard)
On hit, applies a poison debuff that reduces Poison Resistance by 15% for 1 turn. Stackable.
the poison resistance debuff means that your subsequent mutated arm attack effects are more likely to proc. but the best part about this is the crazy crit chance.

10/10. This greatly increases your crit chances and is the best right hand mutation marauders can get. (thanks to moiraine and siema87 for the information, that marauders cant get the ripper mutation which is restricted to chaos spawn and possessed (see below))

Slicer (right Hand Mutation)
Effect: Melee attacks gain the following benefits:
Hit chance +x% (more with purple armguard, not tested)
On hit, applies a poison debuff that reduces Melee Resistance by x% for 1 turn. Stackable.
8/10
While reducing the Melee resistance is nice, insulting the enemy works just fine aswell, and i think the Crusher Mutation is slighty superior to this one. Crits and stuns are just so good. but i guess getting a Marauder with that and then saving the points for master insult might make a decent Warrior too so ill still give it 8/10

Destroyer (right Hand Mutation)
Effect: Melee attacks gain the following benefits:
Attacks bypass +18% Armor Absorption (purple Armband)
On hit, applies a poison debuff that reduces Melee Damage dealt by -9% (purple Armband) for 1 turn. Stackable.
2/10
Well while it may look nice on paper, Targets you are deciding to attack wont survive their round anyways so i dont see any point in this. it bypasses Armor absorption which is nice. but you cast Weapons of Destruction on both your Chaos Spawn and on the Marauder all the time and this already provides plenty of Armor bypass. The other right Hand Mutations are just better.

Claw Foot.
8/10
While its still a great Mutation, and always nice to have, its not as Core as the Crusher Mutation on your Melee guy. when you get this at Level 1 you can try him up to level 5, but if he doesnt get the Crusher or Slicer by then it may end up a waste of time to keep going, since leveling from 5-10 takes alot longer. If he does get it both at level 5 however its just amazing! #thedream. but the odds are probably not that great.
The reason i just rate it with 8/10 is because getting the Crusher basically means you can feel just fine when you subsequently get only mediocre mutations, while this does not hold true for the Claw Foot.
The extra charge range is really nice too for marauder because this makes his charge range higher than the ambush range of most enemies, making it easier for benefit from his passive skill.

Wyrdstone Horns
7,5/10
Better than on your Mutant because he will be in combat more, highly benefitting from the increased tankyness it provides.

Thousand Eyes.
6/10.
Problem with this is you cant use light armor anymore and have to go with Cloth. im havent really tested it much how it relates to your tankyness. whats good is that wearing cloth means your Marauder comes right after your Chaos Spawn.


The Eye
4/10
More initiative and crit resistance but comes along with not being able to wear proper light armor anymore. I personally prefer Armor Absorption over the initiative but if going first with the Chaos Spawn as a double strike team fits your playstyle it might work too. For getting targeted the Thousand Eyes is the better one for sure.

Crown of Bones
5/10
Not ideal but if you already have Crusher on your right arm its fine. He wont get alot of crit/stun resistance from his skills so the stun resistance is actually quite nice. this + amulet/talisman is probably slighty better than wearing a helmet.

Extra Face
6/10
Buffing your Parry chances by 15% is a nice thing, worth 6 skillpoints. you usually wouldnt skill it but if you get it as a Mutation its nothing core but still a good thing to have.

Featureless Face
2/10
idk, i never use perception, but maybe im just an ignorant noob :P

Daemon Soul
5/10
He already is immune to all alone, but 25% Magic Resistance aint bad.

To my knowledge, Marauders cant get extra arm Mutations, so i dont cover those here.
As with offhand Mutations, it means you can attack less often, thus less stacking debuffs. and no more shield makes him alot more vulnerable. so i recommend getting a new one if you get a left hand mutation. the only exception is maybe the Smasher Mutation (see chaos spawn) since its just so good. if you get Smasher in the left hand and Crusher in the right hand, this is surely quite a beast. Also note that if both hands get mutated he cant pick up wyrdstone and other loot anymore.

Recommended Stats

Strength 9 / 16 (higher strength build works too if you wanna see bigger numbers)
Toughness 13 / 15 (12 Toughness minimum for Adrenaline Rush)
Agility 10 / 10

Leadership 12 / 12 (needed for Master Insult)
Intelligence 3 / 9
Alertness 10 / 12

Weapon Skill 16 / 16
Ballistic Skill 3 / 3
Accuracy 9 / 17

Note that i chose to go max weapon skill over accuracy because it improves defensive capabilities too, and crit is buffed with gear and mutations more easily and higher than the chance to hit. and in order to sucessfully crit you need to be able to reliably hit in the first place.

Suggested Skills:

Blood Offering (M) <- skill this first
Swift Charge
Adrenaline Rush (M)
Insult (M)

Fatality
Shield Mastery 5% melee resistance 5% parry is great for cheap 2 points.

I thought about getting mighty charge and strider on him instead of swift charge and shield mastery. but after some consideration i dont think its worth it. because the biggest damage output comes from the regular basic attacks after using blood offering. and in order for the marauder passive damage bonus after charge to work your charge needs to hit in the first place. so im pretty sure swift charge is the more reliable and thus better way to go.
Adrenaline Rush is nice because it gives you a total of 12 Offense Points, making you be able to charge, then use Insult, Blood Offering and then still attack 3 times for ALOT of damage in the first combat round. this makes the posssibility of a stun highly likely.
Also note that the Marauder passive 20% dmg increase from charging stacks with the 50% dmg increase from blood offering so the subsequent attacks hit really damn hard.
Obviously you always want to use insult first if you have enough Strategy Points, then Blood Offering and then just normal swings.

If you dont like Adrenaline Rush so much and prefer a more defensive approach (that still does quite some damage) this skillset is great aswell:

Blood Offering (M)
Defensive Stance (M)

Awareness
Shield Mastery (M)
Resilient
Knowledge: Mordheim or Anticipation

This build works best if you can get a Hired Sword with Thousand Eyes and/or Wyrdstone Horns, which are the most core Mutations for this build. Slicer is maybe a bit better here, because this build has no room for Insult.
A Mutant would probably be slighty better for that build because his extra crit passive after picking wyrdstone is actually useful.

Ok. now comes the most interesting part. From my experience the Chaos Spawn has the potential to be by FAR the strongest unit in the game. If you get the right Mutations this thing can easily kill even other Impressives on Deadly in just a single turn. 250++ dmg swings are no rarity with WoD active.
In order to be able to do so, we once again have to take a look at the desired Mutations. Im trying to list them by priority.

Ripper (aka The Lobster,right hand)
Melee attacks gain the following benefits:
Critical hit damage +15%
On hit, applies a poison debuff that reduces Critical hit Resistance by 15% for 1 turn. Stackable. (purple armband)
11/10
Since your Marauder cant get this Mutation you really want this on your Chaos Spawn so badly. its crit resistance debuff is almost 4 times as good as double runes of misfortune. this is by far the best mutation you can get. the lobster is just incredible and very core. Reliable crits and stuns on ANYTHING FTW.

Smasher (left Hand)
Effect: Melee attacks gain the following benefits:
Critical hit chance +8% (purple armband)
On hit, applies a poison debuff that reduces Stun Resistance by 15% for 1 turn. Stackable.
10/10
This makes sure that your crits guarantee resulting in stuns. If you get Ripper + Smasher on Chaos Spawn this thing is OP. Even Ogres with Helmets are stunned in no time.
Since your Marauder Hero preferably tries to keep his left Arm for using a Shield and being able to loot stuff, ideally you want this on your Chaos Spawn.

Extra Limb: Blade
Effect: Melee attacks gain the following benefits:
Melee Damage +10%
Attacks bypass 10% Dodge and Parry chance.
Critical hit chance +5%.
9.5/10
Great thing to have. Boosts damage tremendously, synergizes nicely with Daredevil.

Extra Limb: Axe
Effect: Melee attacks gain the following benefits:
Melee Damage +10%
Attacks bypass 10% Dodge and Parry chance.
Attacks bypass 10% Armor Absorption.
9/10
Just like with Blade, the extra limbs synergize with Daredevil. Imo the Blade is slighty better than the Axe because Master WoD already gives nice Armor Penetration but its effect might come in handy nontheless.

Extra Limb - Mace
Effect: Melee attacks gain the following benefits:
Melee Damage +10%
Attacks bypass 10% Dodge and Parry chance
Hit chance +10%
9/10
If you get this you can go Accuracy all the Way to 20, 23 with Martial + Accuracy Book. 2% Hit is basically the Equivalent of 1% crit, but Weapon Skill also comes with some Melee Resist so technically its slighty worse than the Blade, but still nothing to say no to.

Crusher (right Hand)
Effect: Melee attacks gain the following benefits:
Critical hit chance +8% (purple armband)
On hit, applies a poison debuff that reduces Poison Resistance by 15% for 1 turn. Stackable.
7/10
While this one means you cant get the Ripper Lobster Claw its still a great Mutation.

Slicer (right Hand)
Effect: Melee attacks gain the following benefits:
Hit chance +x% (not tested with purple armband)
On hit, applies a poison debuff that reduces Melee Resistance by ~18% (?) for 1 turn. Stackable.
6,5/10
Its nice but same with the Crusher, it makes the Lobster impossible. but if you get this as your final Mutation you can still be happy for sure. It makes Insult neglectible.

Destroyer (right Hand)
Effect: Melee attacks gain the following benefits:
Attacks bypass +18% Armor Absorption
On hit, applies a poison debuff that reduces Melee Damage dealt by -9% for 1 turn. Stackable.
5/10
While Arm Mutations are great on Chaos Spawn, this one is surely one of the weaker ones. The poor thing which the proper Chaos spawn choses to attack wont survive the round so the Damage debuff is basically irrelevant.

Executioner (left Hand)
Effect: Melee attacks gain the following benefits:
Attacks bypass +18% Armor Absorption (not tested with purple Armband)
On hit, applies a poison debuff that reduces Armor Absorption by ~18% (?) for 1 turn. Stackable.
7/10
its the lefthand equivalent to the Destroyer, along with some stacking armor absorption. if you get this along with the ripper in the right hand you can consider yourself quite lucky.

Piercer (left Hand)
Effect: Melee attacks gain the following benefits:
Hit chance +x% (not tested)
On hit, applies a poison debuff that reduces Ranged Resistance by 18% for 1 turn. Stackable.
3/10
While it may be nice for the Mutant Ranger its definetely the worst of all the great Arm Mutations.


Ok this covers all the Arm Mutations. You want as many of them as possible but the likelihood of getting 3 of them (one left Hand, one right Hand, one extra limb, or either on left or right hand and 2 extra limbs) is very low. the only possibility to get a realistic shot at this is getting a 5+ Hired Sword Chaos Spawn which already has 2/3 Arm Mutations. Also note that quality is over quantity, the Ripper and Crusher together have undoubetly the best synergy together.
There are other Mutations that are great too.
The Chaos Spawn i settled for has Ripper, Extra Limb: Blade and Claw Foot.
While the Chaos spawn doesnt really need the Claw Foot it gives the Chaos Spawn the ability to be anywhere at anytime. So i would rate the Claw Foot for the Chaos Spawn still with an 8/10 and still take it any day over something like Executioner. It depends alot on your personal playstyle and strategy too. the odds of getting the perfect Chaos Spawn with Ripper, Crusher and an Extra Limb are less likely than winning the lottery. it all comes down how close to perfection you wanna aim for and what youre willing to settle for. As mentioned before, be patient and dont worry too much if you have to fire them. its part of the excitement of the cult and just keep your eyes on the long term goals with the factions. and there is no need for the perfect chaos spawn anyways before you dont even have warpguards imo.

Final Attributes:

Strength 16 / 16
Toughness 12 / 16 (req for Adrenaline Rush (M))
Agility 17 / 20

Leadership 13 / 16
Intelligence 12 / 12
Alertness 20 / 20

Weapon Skill 17 / 17
Ballistic Skill 3 / 3
Accuracy 15 / 20 (18/20 with martial book)

The Martial stats depend mostly on your Mutations.
If you get the Ripper, you can go crit enchantments. i recommend to max weapon skill. (4% crit is more valuable than 6% hit chance on the enchantment as compared to the Weapon Skill / Accuracy ratio)
If you dont get the Ripper you want Runes of Misfortune and then probably max Weapon skill too, unless you get increased Hit Chance Mutations. with Hit chance mutations you can and should definetely max Accuracy.

Skills:

Daredevil (M)
Adrenaline Rush (M)
Blood Offering (M)
Sidestep

Fatality (M)
Knowledge: Mordheim (if you have Claw Foot you can ignore this and go Avoid + Awareness)
Avoid or Awareness

You dont charge with this guy. Just walk up front and personal. against Heavy tanks like Sisters Impressive you go Daredevil, Adrenaline Rush, Blood Offering, 3 Swings.
Against anything non impressive just Blood Offering followed by 3 Swings is more than enough to kill. most already die after 2 swings. even on Deadly. Just keep master WoD from your Leader active at all times.
There is no unit in the game that hits remotely close to this. The reason he is so much stronger than the ogre or rat ogre are not only the Mutations, but also the unique CotP skills. Blood offering 50% dmg and 10% crit chance increase along with Weapons of Destruction for another 40% Dmg increase along with 20% Armor bypass is just incredible. when you kill something walk up straight to the next enemy if you have enough mobility left. so you can use order on your magister on him for another swing and potential stun along with terror.
even though i build my chaos spawn almost full damage he rarely gets targeted by the AI. in pvp you will want to play a little more cautious. but this thing is your biggest trump.

Last but not least lets talk about the Magister.
Hes probably the weakest of all leaders in the early game and quite vulnerable. His direct damage spells require him to expose him a bit too much for my taste. The bread & butter spell is definetely Weapons of Destruction. So max this as soon as you can. your melee hero and your impressive will benefit greatly through this.
As he levels up and you get better gear, he becomes really strong and has a huge impact.
Whats good about Magister is that he is really versatile, there are alot of builds that work with him. you can use him as a Ranger with a Bow. i even heard about heavy armor tank 1h+shield melee build that work around aoe stuns and word of pain. I personally prefer him as a backline buff / order / disable spellcaster and keep the staff on him to minimize curse risk. But feel free to experiment with him and find out what fits your playstyle most.

Suggested Stats:

Strength 3 / 10
Toughness 9 / 12
Agility 14 / 14

Leadership 12 / 18
Intelligence 15 / 19
Alertness 15 / 18

Weapon Skill 12 / 12
Ballistic Skill 5 / 12
Accuracy 10 / 15

Suggested Skills:

Order
Insult (M)
Introspection (M)
Sidestep

Channeling (M)
Knowledge Mordheim (M)
Avoid

Suggested Spells:

Weapons of Destruction (M)
Chains of Chaos (M)
Veil of Torment (M)

Note that this is just one example how to build him. I fail to see anything like an optimal build with him. For example as an alternative, if you want to use Order twice a turn with 2x Weapons of Destruction you can also go Adrenaline Rush and give him 12 Toughness. Though then his dodge chance will be really low if he gets engaged on.
Or you can improve his casting range and give him acid breath. apparently thats supposed to work. right now im having another magister where i try out different setups on him. once i have more ideas about viable builds i might add this here.

My current playstyle with him is to use WoD on the Marauder and Chaos Spawn, insult anything thats going to be targeted, use Order on the Chaos Spawn and if there is an enemy impressive lucky enough not to get focused yet by the Chaos Spawn i use Introspection + Chains of Chaos on him.
Veil of Torment is just a weapon of last resort if he really should get caught. usually there is no need for this.

Here [nithon.de] is another build suggestion for the Magister thats inspired by Slaviour. This time its about stacking Melee Resist on him and thus making him more viable to go along with your frontline.
He uses a Staff(curse resist rune) on his main slot and Mace(curse resist rune)+Shield(w melee resist rune) in the second slot. once you get close to the enemy you swap to that.
Note that he needs alot of Tomes to get all that. If you also want Warcry mastered over Insult he needs a mental grimoire too. Though i would advise to get mastered Insult on at least one character in your team. And if you want to use Warcry + Defensive Stance on the same turn you cant move at all.
you can also skip Veil of Torment and keep his Int at 12. But i think Veil of Torment would be pretty nice with this build because you dont need to shy away from getting into the middle of the action. However if you use 1h+shield its really important that you get your hands on a spellbinding and focus rune in weapon and talisman/amulet slot to minimize your curse risk as much as possible. and casting more than 2 spells per turn without a staff becomes really risky either way. because as mentioned before, if you get stunned through a curse all this melee resist isnt helping at all.
I recommend light armor for that build path and no helmet. In heavy armor he just gets too slow otherwise. In defensive stance you can get up to 96 Melee Resist that way, making him a great tank.

Anyways, thats it for now. Thanks alot for your interest and i hope you find this guide helpful!
If you have any questions feel free to ask. Or if you have suggestions about something i may have missed, id really appreciate it if you let me know. :)