Must-haves are all the passive skills ("Lockpick", "Disarm", "Alchemy", "Bow") and "Bite", "Secret Sight", "Vampire Gaze" and "Mist Morph", as they are crucial to the game mechanics. Everything else, depends on your playstyle. I found "Firecraft" and "Summon Spiders" useful. "Teleportation" + warp stones are useful in the dungeons and dangerous combat situations, but it's not mandatory, as it's relatively easy to find the exits anyway.

3 inventory pages can be unlocked with talents. Quick slots can be used for consumable items. Additional quick slots can be unlocked with a heavy belt. A heavy belt can store consumables both in an equipped and un-equipped state.

Glyphs enhance non-magical items, making them magical and potentially increasing their rarity. Enhancements might include magical stat enhancements or unlocked gem slots.
It's inconsistent and I don't know what determines the type of enhancement that you get. Sometimes, you get a stat improvement, sometimes you unlock one slot, sometimes two or three, and sometimes you can use more than one glyph to enhance an item.
Supposedly, a full moon glyph > a half moon glyph > a quarter moon glyph.

It seems that the purpose of glyphs is to improve non-magical items with strong damage or armor stats so that they can potentially become better than magical and rare items. But, after acquiring unique items, glyphs lose their usefulness and you can safely sell them.

Black orbs are very rare items that also allow to improve equipment items, even those that have already been enhanced (but not always). You can get one black orb around sub-level 20 of the Passage dungeon under Fu Kings restaurant during a quest that you trigger by eavesdropping a conversation between two girls in the nightclub's WC and 4 are behind one of the electrical doors in the main street. If you combine 3 black orbs, you get a black ember that improves your charisma (x Alchemy multiplier).

You can also put one in a statue in sub-level 3 of the Ancient Dungeon, where you kill the Swamp Queen, to activate a portal that will take you deeper in the dungeon. (It was sub-level 3 for me, it might change every time you enter the dungeon).

To insert a gem, click on it and right click on the item you want it to be inserted in. The colour of the slot has to match the colour of the gem. To take the gem out, press Alt+click. Sometimes, the gem is destroyed during the removal process. I don't know if it's a bug or it's supposed to happen. Note: different gems have different quality (hover over them to see it).

• Orange ghoul eye: increases max armor.
  
• Red rat eye: increases min damage.
  
• Purple raven eye: increases max damage.
  
• Blue monkey eye: gives shock damage, serves as keys for fast travel gates in dungeons (2 or more required).

Some gem slots have a different shape (a wide irregular hole) and you can't put the aforementioned gems into them. I have never found a gem that I can put into them, and my guess that they are just "destroyed" slots OR somehow these items can also use Odios glyph (I haven't tested it), which has a similar shape.

I don't know if a set of matching items provides a bonus (probably not), but I have encountered following sets:

• Apprentice
  
• Sorcerer
  
• Wizard
  
• Emperor

I have encountered items of following rarity:

Common/Physical < Magical/Enhanced < Rare < Unique

Locations copied from this guide: http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=403782830

• Dungeon: the courtyard (the starting area)
  
• Passage: Fu Kings restaurant
  
• Ancient Dungeon: the tattoo shop
  
• Egyptian Dungeon: the hotel
  
• Cave: the west sewers through to the east ruins

Things that you can find in a dungeon:

• Loot – the deeper you go, the better loot you find.
  
• Enemies - the deeper you go, the stronger are the enemies. The stronger are the enemies, the faster you level up.
  
• Chests, secret rooms, traps and puzzles.
  
• Alchemy stations - for mixing potions
  
• Anvils - for repairing badly damaged items (durability of 2 or less).
  
• Triggers - a yellow square on the map.
  
• Stairs to the exit - a red square on the map. At least one per every 5 sub-levels.
  
• Chambers - a pink square on the map. This is where you find bosses (Murdock, King Vuul,...) and an exit from the dungeon. They are not very strong.
  
• Treasure rooms - a red square on the map. Contains a secret room with potentially rare loot that you can unlock with a code that at some point spawns in your inventory (you might exit and re-exit the room to spawn it). It also contains an exit from the dungeon.
  
• Fast travel stations - a blue square on the map. Allows to bypass about ten levels or more.
  
• Cracks in the walls - a red square on the map. Allows to bypass about 5 levels.
  
• Pits - a purple dot on the map. Allows to proceed to the next sub-level.
  
• Potential minions: Sally, Miner, etc.
  
• Paco - a trader (sells expensive, buys cheap), with enough persuasion you can get access to his special stock once per (character's) level (at least 300$ required.) Paco can be encountered on sub-levels with exits between 1 and 30.
  You can exploit the trading system, by having enough stakes in your inventory. All sellable items cost no less than 1$. So, you can sell a few stakes to Paco's special stock and then sell all of them with Shifth+click. (You can't sell them one by one, because there's a limit of about 100). Thus, you always will be able to afford something from his special stock. But for this purpose you have to invest in "Bow" skill. To get some stakes back, buy them from Paco's regular stock or take them from your minions (about 10 should be enough). Then farm more, by killing the enemies.
  
• Vico - another trader below sub-level 30.

You can teleport out of the dungeon with the warp stones, which you can purchase from Paco. With every point in "Teleportation" skill, you can use them one sub-level deeper below sub-level 1.

Locations copied and edited from this guide: http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=403782830

The hideouts are guarded and you need to have a good reputation with the clan to travel through them openly, unless you don't mind being attacked. But, if you are doing something that you aren't supposed to be doing, you'll be attacked anyway. And the guards will be following you no matter what your reputation is, which kind of blows. Fortunately, you can kill the guards and it has no effect on your reputation with the clan. You can increase your reputation with the clan by doing quests for them or stashing clan items in their stash trunk. Tattoos from the tattoo shop can alter the reputation gains with specific clans. If you haven't noticed yet, skills have a reputation requirement.

Maps of the hideouts can be purchased in the delivery shop (access with coupons via the pay phone).

• Scades: the tattoo shop
  
• Vurkas: the night club
  
• Vangres: the pawn shop
  
• Furlores: sewers under the main street, outside the courtyard; another entrance is in the area in front of the hotel, not far from where a girl is talking on a phone. The key to their hideout can be pickpocketed from one of their clan members in the theatre during one of the Vangres quests. I don't know if there's any other way to get that key.
  
• Shadistin: the massage parlor. They have also another stash behind a wall in Fu Kings restaraunt. You open the wall, by turning the valve on the pipe.

One lockpick is moved with mouse to the left and right.
The other one is moved with mouse up and down.
When you find the sweetspot, press LMB.

You need a drill and drill heads to crack the safe. Press white squares, until the safe opens.

Level 10 alchemy is required to unlock all the recipes. The better your alchemy skill is, the bigger the bonus that you get from potions. All the recipes are listed in the alchemy tab that you can access with alchemy tables or by right clicking an empty bottle. The most useful recipes are for alchemy oils that improve your stats:

• Charisma perma-boost: empty bottle + venom sack
  
• Strength perma-boost: empty bottle + green roach + elixir
  
• Vitality perma-boost: empty bottle + snake scale + elixir

Snake juice is not working for some reason.

Keep a bit of every alchemical ingredient – you never know for which quest it might be needed.

Three zombie hearts allow to craft a piece of ember, which doubles the vitality gain of "Alchemy" skill.

Items can be repaired with:

• Blacksmith oils, which can be purchased, looted or mixed (empty bottle + blood wine + stamina boost)(stamina boost can be purchased, looted or mixed = empty bottle + shadow weed). Blacksmith oils repair the items partially.
  
• Repair tools (can be purchased from the delivery shop via the pay phone)
  
• Anvils. To use them, the equipped item should have durability of 2 or less.

Items can be enhanced with glyphs (non-magical items only) and the enhancer, which is located in the back alley behind the tattoo shop. Enhancing with the enhancer costs game tokens.

Items can be recycled to obtain a potentially better random item in the recycler, which is located in a corner down the alley to the left of the pay phone and past the nightclub near the west sewers. To recycle you need a minimum of 3 and maximum of 9 items. 9 items give a 90 % chance of a rare item. It doesn't seem that recycling rare items provides unique items. So, if you have unique items, there's no point of recycling anymore.

With "Bite" skill of 15 you can sire maximum of 4 minions, which in turn can sire their own vampires. It doesn't seem that there are any moral repercussions for siring minions. When your family size has reached 20 members, you gain access to the penthouse suite in the hotel. The penthouse is a place where you can stash your items, mix potions, repair and improve your items and find the vampires that your minions have sired. To kill your minions, place 3 cloves of garlic in their inventory.

Your minions are of limited use and you can easily do without them most of the time. I mostly use them as meat shields. Therefore, I primary invest in their vitality and strength. But you can also use them to search for consumable items (lockpicks, stakes, elixirs) and expanding your family (charisma). Every 15 levels of dexterity increase their speed. You can also use the minions to activate pressure plates. It seems that, in some places, it's the only way to activate two pressure plates at once.

Locations of potential minions (copied and edited from here: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1187541328)

• Walter - the bum sleeping in the ruins courtyard; the most powerful minion (?)
  
• Fred - the bum sleeping in the ruins proper, near the hidden door to the alchemy area
  
• Gonzalo - in the sewers
  
• Nina (goth girl) - in front of the tattoo shop near the sewer
  
• Johnny - walking near Nina
  
• Jess - outside the asylum, talking on the phone
  
• Tony - in the city under the train tracks
  
• Hank - in back alley behind the tattoo shop
  
• Banger - in Fu Kings
  
• Security Guard - located in the hotel
  
• Emily - in the warehouse
  
• Nina (different Nina) - random dungeon area
  
• Miner - random dungeon area
  
• Sally - random dungeon area

NB! You can feed on rats btw.

You will need the ancient dagger to access all the odios glyphs. The ancient dagger (dagger of the ancients) is located under the cinema. You will have to solve the pressure plate puzzle to get it.

(Copied and edited from this guide: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1187541328)

1. Fu King's restaurant - use the barrels on the platforms to activate the pressure plates in the snake statue room.
   
2. The Vurkas hideout (the nightclub); in the sarcophagus behind the painting.
   
3. Hidden under the bar, behind a secret wall with a panther guarding the inside.
   
4. Hidden under the bar office, behind a silver barred gate.
   
5. Hidden in the Scades hideout (the tattoo shop), in the sarcophagus in Rojo's room above the waterfall - pick the lock on the door to extend the bridge.
   
6. Hidden in the Scades hideout, behind the door with a face and the passcode is hidden in the secret bedroom ("chamber")
   
7. The Vangres hideout (the pawn shop); use the ancient dagger from the theater on the face. Make sure to use the random three letter code found on a note in one of the guard patrol rooms.

Blood fragments are relatively easy to find. They are scattered troughout the map behind secret or electric doors. To find them you will need "Secret sight", high hacking skill (intelligence) and the shadowhunter breed that allows to access "Shadow dust" skill. You can find a few in each hideout or other special locations (about 4 or 5 in each), in the first ruins (5, if I remember correctly), a few in the east ruins and a few in the main street, behind electrical doors. Without "Shadow dust" skill, you won't be able to collect all of the blood fragments (e.g., the fifth one in the ruins), which means that it's not possible to collect all blood fragments before finishing the game. If you really want to bother with checking up with a list, this migth help: https://psteamcommunity.yuanyoumao.com/app/280600/discussions/0/351659808475804464/

1. What are the songs in the tattoo shop?
   
   "Hard Flirtation" and "Takin' Deep Breaths" by Ruby Red.
   
   
2. Where is Victor's lab?
   
   It's in the basement of the hotel. Take left after exiting the lift and look for a column with a hanging chain. There should be a barred opening near the floor. Use "Vampir gaze" on it.
   
   
3. Where is the code for the third moon lense?
   
   In the room with the panthers, there's an opening in the wall behind the throne. Use "Vampire gaze" on it. Level 1 "Vampire gaze" will not be enough.
   
   
4. What the statue near the Swamp Queen does?
   
   It opens a portal, if you drop a black orb near it. The portal can take you deeper in the dungeon.
   
   
5. What does the second door lever in the dungeons do?
   
   It ensures that you can return, even if the first door is closed.
   
   
6. Where is Green Eyes (Mari)?
   
   After fighting her a few times in random dungeon areas or elsewhere (preferably, with a melee weapon), she will initiate a conversation and drop a key to the bar office. You will find her in the Council's chamber, where she will give you the sword of Odios.
   
   
7. How do you backstab?
   
   Supposedly, you sneak upon an unsuspecting enemy and press RMB (special attack, I don't know if it definitely has to be the jump attack). I have never managed to kill anyone with it, so I don't know, if it actually works.
   
   
8. How do you pickpocket?
   
   Supposedly, you sneak upon the Furlores guy in the cinema and then a prompt "pickpocket" appears, but it didn't work that for me. You can distract his guard with a firecraker. As long as your flashlight is off and you don't run, they will not notice you.
   
   
9. What happens if you collect all blood fragments?
   
   I don't know, I was too lazy to bother with the last fragments after the end of the game.

Traces around your character: reset the video settings to default.