So you've gone through the grueling process of hunting down and capturing an Alien Praetor and you have interrogated him, after you close this research window another thing pops up saying "new projects available" and below that, operation endgame, you will want to start researching this asap since it unlocks the final mission.

After research on that has finished the game will not be put into and endgame state officially, (unless you count the numerous battleships that will appear on the map with each UFO wave as an endgame state) instead a bit of text will now be at the top of the screen which says operation endgame, if you click on this you will start the mission and there is no turning back once this is clicked, so do not click it until you are ready.
What you want to do now is make sure you have the following things researched: all forms of MAG weaponry, Fusion Explosives, Predator Assault Armor & Sentinel Battlesuit, Valkyrie Dropship, & finally the Singularity Cannon, other optional items include the Hyperion Hovertank, & the X-120 Fury Interceptor, while the X-120 is powerful it won't leave a wreckage so if you want to keep the dough rolling in it's best not to use this interceptor except for high profile targets like UFOs on terror missions, as for the hovertank, it does have it's uses but more often than not it just gets in the way of things so I wouldn't think it's necessary unless you really like the armored vehicles in this game.

Here's what the vehicles look like in case you're wondering

Now what to equip your soldiers with. You will need the Valkyrie Dropship to carry as many soldiers (or equivilant) into this mission, For the most part you're going to want 12 soldiers, 6 equipped with predator armor and 6 equipped with sentinel battlesuits, the latter for scouting and sighting the enemy and the former for mowing them down. As for weapons the predator side is very simple, you'll want 5 of them equipped with MAGstorms and 1 equipped with the singularity cannon, if you're taking the hovertank you can replace the singularity cannon specialist and 1 of your MAGstorm troopers with the tank, if you have the armored assault mod installed I'd recommend taking the fusion rockets to make up for the lost firepower but the MAG Railgun could work.
As for the sentinel side of things you will want 2 as snipers, 2 on assault, and 2 as riflemen, by this time sheild troopers are basically useless so there is no reason to include them unless you're insane or a genius. If you have chosen the way of the singularity cannon then you will need to give everyone the slugs for the weapon because otherwise your specialist will not have enough TUs to fire and reload the weapon even with predator armor, but it's ok to give him/her 1 or 2 slugs to start off with, and don't forget your grenades!

I've provided a couple of examples below of what your soldier's loadouts might look like, the ones I have here are currently equipped for base assault instead of operation endgame but swapping out the plasma charges for slugs should work well enough


After you have chosen your strategy if any of your soldiers have stats less than 80 you will want to train them on crashsites (as you will have virtually unlimited UFOs descending everywhere there should be no shortage of them) until they are above that level, if you can I heavily recommend maxing out the TU stat, you'll want as many as you can get, however if the previously mentioned stats are Bravery or Reflexes then you will have to leave them as they are because there is virtually no way to up those stats without interacting with the enemy and as you tend to lose soldiers interacting in the manner that upping the stats will require, it's better to leave them as they are I completely forgot psionic attacks were a thing so if you want to try to train up the Bravery stat send your soldiers to crashsites with caesan crews, anyway if you don't know how each stat increases here's a quick run-down:

• Time Units (TU): Gained by spending a set amount of TUs on a mission
  
• Health: Increases based on how many other stats increase in that mission
  
• Strength: Gained by spending TU while over 80% carrying capacity
  
• Accuracy: Gained by firing at an enemy, actually landing a shot isn't a requirement
  
• Reflexes: Gained by getting a reaction shot at an enemy, again actually landing the shot isn't a requirement
  
• Bravery: Gained after a soldier suffers a morale event like a panic
  
• Rank promotions are earned after a soldier gets a certain number of stat increases throughout their career, each promotion providing a general buff to stats
  
• Medals are awarded for completing certain actions during a mission and each badge gives +1-3 Bravery, I don't know exactly what each and every one is but most are for killing a certain number of aliens or participating in certain missions i.e. base defence, terror, etc., I'll leave a link here if you want to check out the full list: https://xenonauts.fandom.com/wiki/Medals

Now before you begin Operation Endgame you will need to save the game in case of bugs that can occur or in case you mess it up, if you're playing Ironman mode then the save function wiil be unavailable to you so all I can say is good luck.

After you have clicked on operation endgame a screen will pop up and you will be given the option of what dropship to use, you will need to select whatever ship you have put your A-team on and click launch, after that you will get a screen (shown above) detailing the mission which is simple, kill the high praetor. However the problem with the mission is A) If you march directly for the Throne Room then you will not be able to double back for the fusion cores which you need to destroy to activate the escape pods, or at least it will be extremely hard since reapers will be coming out of the vents by then, and B) you have one shot unless you saved beforehand (which is recommended) so again, make sure you're thoroughly prepared, after you close this window the mission will begin. Once the mission starts during the deployment phase you will want to put your soldiers as close as possible to the door in this order: Sentinel, Predator, and if chosen, Hovertank then click end turn, this will not start the alien turn just simply give you your starting turn exactly like an alien base assault.
You will then want to open the door and obliterate all aliens in sight with extreme prejudice, after this the real battle begins. Depending on what loadout of soldiers and/or vehicle you have chosen you will want to divide up your troops in to 3 different squads for optimal effect, Assault and Riflemen Snipers along the sides with 2 predator troopers on the right and left sides to take care of the fusion cores, while the 3rd squad should contain 2 Snipers Riflemen and either the hovertank or a predator trooper and your singularity cannon specialist, the idea being the snipers are good for the long corridor to get the full use of range on the enemy and the singularity cannon or hovertank being the powerful weaponry to annihilate whatever alien is left. while this looks like a good plan on the surface you're better off sending the snipers in to deal with the fusion cores because of the aliens that guard the cores, the predator layout is optimal though.

So once you have set up your squads you will want to send your side squads into the side doors that appear, you will then run into a mix of Sebillians and Androns, you should know how to deal with these guys by now but if you don't, all you need to do is make good use of cover and range, since sebillians have poor accuracy and androns don't take cover that is your best counter, you will have to place your assault troops closer for good effect so have them deal with the androns whilst evading the lizards, while your riflemen snipers take out those scaly dudes and use the predator troopers to mop up the rest.
As soon as you reach the door to the room with the fusion core have one of your troopers shoot the core, one well-placed shot from a shotgun should be all that's required to make the core explode, but any firepower will do. Once this is done have your Fusion squads join up with the main squad as quickly as possible, it'll only take 6 turns for reapers to come up behind you so you need to move as fast as you can, this is where the maxed out TU stat comes in so you can move your squads as far as possible in one turn.

Meanwhile the main squad you have set up should be moving down the main corridor, all you need to do is eliminate any alien you come across. there is a heavy hover drone almost directly after the first door in the hallway, take no chances with this thing, obliterate it with the singulairy cannon or the hovertank, if you can't get a clear shot with either of those then fire at it with whatever's available while you move the heavy weaponry into a suitable location to take that thing down, trust me it's extremely powerful and you don't want it to so much as glance in your soldier's general direction.
After the drone is dealt with there are a couple of vents on the wall before the door, use your assault troopers to blow open the vents, then have them circle around with some riflemen support inside the vents to the next room, and inside will be a ton of caesans and some harridans, again you should know how to deal with these guys by now, since caesans and harridans act like soldiers and snipers respectively, getting closer and flanking them with the guys in the vents and laying down suppression fire from the main entryway should take care of them for the most part, if you haven't got eyes on them yet you can blind fire in the general direction of where the shots are coming from with your predator troopers. After that there is one room left, the throne room which of course contains the high praetor and his guards.

So there is one room left, the throne room, what you need to do now is simply kill the high praetor, which is easier said than done, his attacks are explosive and he will almost always have a reaction shot ready so be careful when moving your troops up, and while you do that make sure to focus as much firepower on him as possible, the sooner he's dead the better.
By this time reapers are climbing up the vents so have a couple of guys looking backwards with TUs for reaction fire. After the praetor is dead all you need to do now is just book it to the escape pods at the very end, send your guy with the most TUs to open the door first then just run everyone else in, as long as one soldier makes it you should be fine.
Once all is said and done you will get the usual mission over screen and after that the victory screen, while humanity faces an uncertain future you have saved the world from total annihlation thanks to your tactical skills and quick thinking. HOWEVER while assaulting the throne room you will need to make sure not to damage the escape pod doorway, there's a bug where if that door takes a whiff of damage it will be in it's damaged state meaning you can't open the escape pods, this wouldn't be so bad if it weren't for the bug where the door becomes invincible after that one hit and with the way the game is programmed if all your men die you will lose regardless if the high praetor is alive or dead, doesn't make sense seeing as by now the dreanought is diving headfirst into the earth's surface and any occupants onboard would be dead after a crash like that but anyway, this bug has occured most commonly in installs with the community edition so straight vanilla should be fine for the most part I have since learned that this bug has been fixed in the community edition but I still advise being careful about who's shooting just in case that bug rears it's ugly head, I haven't been able to try it myself but bringing a plasma charge might blow open the door in case this happens.

Tip #1- ALWAYS USE COVER. I cannot stress the importance of cover enough, any soldier left out in the open will always be shot at, whether or not he/she survives is debatable, especially with the amount of aliens on the dreadnought.

Tip #2-Deal with the heavier weapons first. This applies mostly to aliens equipped with the plasma cannon, those things can cause extreme amounts of damage and should be top priority.

Tip #3-Deal with closest enemies second. This isn't as much of an issue as the heavy weapons but it's still something that takes high priority as scouts can look ahead for all the other aliens letting them shoot at your troops.

Tip #4-Sentinel goes first. In most cases it's better to scout with the sentinel than the predator as the predator has an extremely limited view cone while the sentinel has a 360 degree view.

Tip #5-Watch your back. Considering reapers will start climbing up after 6 turns it's a good idea to keep a couple of guys at the back as a rear guard to tell you when the reapers are crawling up, if you've been following this guide these soldiers should be ones from the fusion core squads.

Tip #6-Smoke grenades save lives. Unless you're fighting against sebillians and androns in the fusion cores smoke grenades will always reduce the enemy's accuracy and visibility, so for example, if you have a soldier caught in the open with no TUs left, toss a smoke grenade to give him/her a better chance of survival.

Tip #7-Bring as much as you can carry. Remember this is the last mission, so don't be afraid to load your soldiers up with as much ammo, grenades, and medkits as possible, I'd recommend ditching stun weapons though since you won't be capturing anybody and more often than not stun weapons take more time to do their job, time which you don't have, that being said the electroshock grenade is an excellent tool for dealing with the Androns and other robotic units like the heavy drone.

Tip #8-Blind firing is your friend here, since the majority of this mission is one really long hallway don't be afraid send a hail of MAGstorm bullets down the hall and potentially suppress aliens you can't see yet.