The Day the Sky Fell (Bureau)
Now aboard the Avenger, make preparations to assault the Mothership. Of Wier, Faulk, and Weaver, there's dialogue with the two humans you did not choose, so feel free to talk to them. Go to to console, choose your agents and weapons, and start the final mission.
It's eerily empty for most of the way: you'll eventually see some retreating Sectoids. Following them, you'll get a small ambush from some Silacoids after a one-way drop, then catch up on the Sectoids in a winding room. Then going up an elevator, you'll be informed that Carter has escaped, and is after the Ethereal -- you -- though he doesn't know which human is bonded to you. At the top, you'll reach the energy core of the Mothership. When you approach the door to the backup Mosaic Core, Carter will set off the alarms: the Zudjari begin attacking in earnest now (so beforehand, if you brought an engineer, this is the last place to set a minefield). They will move in several waves from the teleport pad near the core, and include many high-ranking Zudjari, plus one Heavy Sectopod in the last wave. Watch your sides for Elite Phantoms going around for flank attacks.
Once done, the other two humans radio in for support: you need to choose which one you will help. They will fight as well, and will assist a bit in their path. Angela will have a Plasma Sniper Rife, Myron will have a Plasma Assault Cannon, and Alan will have the Lightning Cannon. Whichever of the two choices you decide on, William will go to the other, and will be there with a Pulse Laser Rifle. There will be an icon for who's in which direction. For storyline purposes: it doesn't matter which you choose, so make up your own head canon on who survives to carry on XCOM's legacy. For tactical purposes: all off them fight autonomously, so you don't get extra commands for them. Both have Supply Drops at the end of the path, so the one you don't choose to go for first, you'll have access to agent and weapon changes to suit better if you need to.
Either way: head for one of the doors. The one with the hologram of a drop pod for Deployment, a Sectopod for Manufacturing.
The door opens up to a desk for cover, and from the bridge in front of it will come two grunts. Head on, and a platform will carry you to the Deployment Wing while a radio conversation plays. On the other end, open the door: you'll notice a teleport pad to your right.
Head onwards down the stairs and ambush the two idling Grunts. The sounds of battle can be heard: be prepared to fight as soon as the door opens.
The area is a large Colosseum, with a high ground and a low ground area. Immediately, you're beset with Elite Snipers, Elite Phantoms, and an Elite Muton. Take out this wave, and the far doors will open, releasing several more ranking Zudjari, and two Elite Mutons: keep a Mind Control for one of these to even things up, or at least a brief respite. It's easy to run low on ammo in this room: there are a few scattered Energy Packs and weapons, scattered more towards the far end. As the Mutons will probably be the last aliens standing, just keep your distance and lead them around the area if you need to go for the packs.
With that done, go through to the back area.
The door opens up to a desk for cover, and from the bridge in front of it will come an Outsider Elite. Head on, and a platform will carry you to the Deployment Wing while a radio conversation plays. You can see the Moon from here. On the other end, open the door: you'll notice a teleport pad in front of you.
Up the stairs, another desk. There's an explosive canister, and a Grunt will soon walk near it: shoot the barrel to get the drop on him, then nearly any other attack will down him while he's exposed. A short turn to the left shows another grunt, take him out too. The sounds of battle can be heard: be prepared to fight as soon as the door opens.
You'll see a small area, where they just soloed a Sectopod, while fending off several varieties of Elite Outsiders. Join the fight, keeping an eye for flanks and grenades, but put priority on that Minion Elite, who's drones can be especially distracting with Lift beams. When that's done, notice the pool of gunk, then head downstairs. You'll see another pool of Silacoid gunk, and when you approach it, another Sectopod will appear--- it's basically trapped in the pool, so don't worry about it jumping out at you.. If you have a recon with Decoy, have the Decoy go behind the pod so you can all get flanking shots into it's plasma conduits, and it'll go down easy. There's even a spare Baster Launcher to help you out.
Moving forward around the second pool, you'll see a third pool, another Sectopod (also effectively stationary), and the doors in back will open, with several more elites, including a Psion Elite, who will share priority with the Sectopod: the same Decoy trick will work a little (for as long as it last under the barrage), so follow it up with the Recon's bombardment, and some grenades.
With that done, go through to the back area.
For the first path you choose, you'll secure the area by activating the console. The person you rescued will stay to deactivate the relay when the time comes. Use the supply drop, then use the teleporter. For the second path, you'll start at the aforementioned teleport pad of that wing.
For the second path: you'll find the body of the other person. Carter will stay to deactivate the relay.
Using the teleport pad again, you'll be back at the energy core. Deactivate the terminal to bring down the barrier, then Carter will catch up with you. You will then have the choice to either kill him, or walk away. Again, for storyline purposes: it doesn't matter which you choose. You decide who carries on XCOM's legacy.
Take the last elevator up to the last Supply Drop crate. Choose your agents, packs, and weapons. The last room is very large, so long-range combat is must. It's an elevated area around the sides, sunken in the center with a bunker in the middle. Plenty of cover either way, so you can either close in to attack, or pick off enemies from afar. It's easy to get flanked, so keep an eye on enemy positions. Finally, the backup Mosaic Core lies behind a final shield at the far end.
Wandering around are several Elite Phantoms, Elite Snipers, and Elite Outsiders, and a Tech Commander who will put up a Turret if not defeated soon. Then another set of Scouts and Elite Phantoms. After them, 4 drones (some Advanced) will be released from teleport pads on the corners at the far end- you might want to let one live for a bit, if you need to let your abilities recharge. Destroy the Drones, then it's another mixed wave of Elites and Scouts, plus a Minion Elite from the same pads. Kill them, and then another mixed set 4 Drones (and again, good chance to recover abilities. Finally, the last wave, with Scouts, mixed Elites (including another Minion Elite and a Tech Commander, again the priority targets), and two Elite Mutons. Immediatley Mind Control one of the Mutons to make the most of while they are all still near each other. The Mutons will almost certainly be the last aliens you kill in this wave. Either way, when it's just one alien left, start making your way to the Core, for when you kill that last Outsider, a console near the core will open, and an infinitely-spawning stream of Mutons and Elite Mutons will pour out of the teleport pads behind you. Use the console to bring down the last barrier, and approach the Mosaic Core to delete Origin and finish the war.