Priority: Cerberus Headquarters

The Illusive Man's headquarters has been located, and the Alliance is prepared to attack to recover the Prothean VI that holds the key to the Catalyst. Once the attack is launched, the Alliance will be fully committed to the final fight against the Reapers. Assault the base when ready.

Acquisition

 * This mission is acquired immediately upon completing Priority: Horizon.
 * After starting this mission, you give up free roaming the galaxy and must go straight to next destination. Finish any other missions you have before attempting this one.

Preparation
When you attempt to dock at the Illusive Man's station, Hackett will talk with you over vidcomm, mentioning that this attack will expose The Crucible to Reaper eyes and must be considered the first phase of the assault on the Reapers themselves. He will give you the chance to back out, which you should take seriously: this is the game warning you that you are about to hit the point of no return.

If you start the mission, you will not be able to complete side missions, shop at or visit the Citadel. You can however use the Normandy store. Furthermore, if you complete the game and then re-open your save, this is where you'll find yourself. If you've got a romance going, that character will come and visit you in your bunk. There's even another ghosts-and-forests nightmare. It's the beginning of the end.

In other words: got anything else you want to do first? Do it before hitting Cronos Station.

Squad
EDI is a required party member for this mission. She is based loosely on the Engineer, giving her lots of tech options but limited offensive capability. As such, you'll likely want to bring at least one character who sources a lot of damage. Crowd control is always useful. And there's a boss character against whom anti-shield powers will come in very handy, with crowd control a close second. Alas, EDI only provides anti-shield tech, so choose your second member based on what Shepard doesn't provide. (The one character who has abilities in all three areas is Ashley; if she's dead, James and Garrus are good fallbacks.)

Consider putting EDI in her "Power Recharge Time -25%" outfit. You may find you'll be using EDI's Overload a lot.

Shuttle Bay
You'll land in a shuttle bay for possibly the hardest fight of the mission: standard Assault Troopers, Centurions and an Atlas Mech will flood in from in front and the right, while a Nemesis sniper camps out on the raised balconies encircling the room. The only reason she can't hit you from behind is that she doesn't have a space suit. Keep your shields strong, wait for openings, and don't lose your nerve. A Javelin, or other sniper rifle with scope mod, might be a good choice here, because one of your main obstacles is simply Centurion smoke screens, which put a fierce damper on your rate of (aimed) fire. If a sniper rifle messes with your weight limits, never fear: there's an M-37 Falcon in the back of the next section (after destroying the wall), which you can exploit to swap your loadout after the fight. Finally, be aware that you don't actually have to kill the Nemesis: your next step is to head to the left side of the room and hike up a ladder to that balcony, and she'll either flee or de-spawn once you do.

There is a time period of about 85 seconds from completing the initial battle and disarming the hanger venting sequence from the upper level terminal. A Critical Mission Failure will occur if you fail to disarm the hanger venting sequence in time, even if you happen to be in the middle of the disarming cutscene at that moment.

Up on the balcony, there's nothing in the little chamber to the left, so turn right and head through the door. After the cutscene, you'll have to fight your way around to the balcony on the other side of the room to use the "Rotation Controls". Look for a medical station in a dark room opposite to the "Rotation Controls" console. Go through the next sets of doors, then, before heading down the ladder, go through another door to activate a PDA for 5,000 credits. Take the ladder down and operate the console to your right, the one with the ostentatious red bars like a jukebox. Admire the boom, and then get ready to fight the Atlas you saw in the cinematic. In addition to that one, there's two more in side alcoves on the left and right sides of the room. The one on the right you can very easily hijack; once there, kill any infantry attempting to grab the left-side one. If you're lucky, a Centurion will pop a smoke-screen and shield you from the only manned Atlas, allowing you to mop up the foot soldiers first.

Through the under-area
Hop the wall in front of the hole and then take the door on the left, being sure to snag the Falcon assault rifle referred to earlier. Once you've gone down the ladder, immediately sprint for cover, as Cerberus troops are waiting for you. You can flank by going forward from the ladder and taking the loop around and back, but if so you may run smack-dab into the Nemesis in the squad. Once you've cleared out the zone, find a medigel dispenser and salvage a terminal for 5,000 credits on the far side near the shiny ammo box to the left of the ladder you need to climb.

In the next room, you will find the first of four terminals, which give you insight into two of Cerberus's biggest programs: the Lazarus Project, and EDI herself. While not objectives, the information here reveals much about Lazarus and EDI, as well as about Kai Leng and the Reaper-tech Implants Cerberus has begun using on its troops, including the time after the events of Mass Effect 2. Serrice Council Greaves is behind you while you're viewing the Lazarus Project console.

Once you're done with this first terminal, head out and into the burning section. There'll be a drop, after which you should turn right; it's another spot where the way forward isn't visible until you're already taking it. You'll immediately be flung into battle with a bunch of Combat Engineers, turrets and Shield Pylons. More often than not, they'll have set up on the other side of a blind corner, forcing you to make a run for cover before you can start hitting the Shield Pylons. Note that Overload (and its cousin Energy Drain) will destroy a Shield Pylon in one hit, and a turret in two if the Engineer isn't able to scuttle over and repair it, making EDI worth her weight in gold.

The Human-Reaper
The top of the ladder yields another lore console, this time revealing that EDI started out as the foe in UNC: Rogue VI. Go through the door, find a medical station and salvage 5,000 credits from a PDA while talking to Hackett as EDI opens the next door. Once through the two doors, go across the hall, ignoring the glowing blue hole in the wall, to find a Medkit, a Delumcore Overlay and the third lore console, about Kai Leng. Once done, backtrack to that blue hole and jump down.

On the other side of the drop, you will find the remains of the Human-Reaper Larva that served as the final boss of Mass Effect 2. If you destroyed the Collector Base, only its heart will be present; if you spared it, much more of it will be there.

You must now fight your way up the catwalk surrounding the larva's corpse. There are Nemeses, Phantoms and really inconveniently-placed Shield Pylons. Take it slow, keep your shields up and don't be afraid to backtrack: things you can't hit from your current position may be open to other angles of attack.

There is the possibility of confronting two named enemies. If Shepard chose to turn Legion over to Cerberus after its acquisition during the Reaper IFF mission in Mass Effect 2, it will appear here as an enemy. Furthermore, if Shepard did not complete Grissom Academy: Emergency Evacuation, Jack will appear here as an indoctrinated foe. Both characters must be fought and killed if met under these circumstances.

The final battle
After going up a ladder right before leaving the catwalk, turn around away from the door for a medkit and 5,000 credit PDA before going through the door. Through the door, you'll reach two long hallways. The first is broken up by computer terminals and has an ammo box, medkit and PDA worth 5,000 credits and a video log nearby about the Cerberus Reaper implants. The second path leads up a ramp to the Illusive Man's inner sanctum, a location the player will undoubtedly recognize. The Illusive Man himself is not here, but Kai Leng is, and needs to be defeated.

As before, Leng stops to regenerate his shields on a regular basis, making it difficult to damage him significantly. He also uses occasional heavy-melee attacks. These attacks can kill you instantly unless you block them by tapping the key allowed to melee. He can summon Cerberus minions, consisting of Assault Troopers, Phantoms, and Nemeses. Focus on Kai Leng except when Nemeses and Phantoms show up.

After he's defeated, there will be a Renegade interrupt opportunity as Leng sneaks up on you and attempts to stab you. If you don't use the interrupt, Shepard will dodge; if you do, Shepard will block the blade and shatter it. Immediately after, Shepard will follow it up with an omni-blade attack, putting an end to Kai Leng once and for all.

You will then have a last conversation with the Prothean VI with an option for a renegade interrupt, or if you wait, a paragon one. Finally you will return to the Normandy for a debrief with Anderson.

Enemies

 * Assault Trooper
 * Atlas Mech
 * Centurion
 * Combat Engineer
 * Guardian
 * Kai Leng
 * Nemesis
 * Legion Assassin
 * Phantom
 * Jack
 * Turret

Aftermath
Shepard will speak to the Prothean VI. There will be an opportunity for a Renegade interrupt, which makes Shepard end the conversation, or a Paragon one, which reassures and sympathizes with the VI. There is no further dialogue if you don't take one of them.

Shepard now returns to the Normandy and speaks with David Anderson, who confirms that the Citadel is now in orbit of Earth. The mission Priority: Earth becomes available. Note that no other system is available for visitation at this point; the only system you can fly to is Sol.

You do, however, have free run of the Normandy herself, and you should feel free to check in with the crew and hear any final comments they might have. Visit Cortez's shopping depot and make any last purchases. Do Mass Effect 3: Galaxy at War activities to do some last-minute buffing on your War Assets; note that you've received either the Human-Reaper Heart (100) or the Human-Reaper Brain (110) as War Assets augmenting the Crucible. Finally, if you did not romance Tali and Garrus, make sure to check the gunnery room to catch them "in the act".

If you sided with Engineer Daniels during her argument with Engineer Donnely, be sure to check the bottom port area where Jack resided in ME2 to catch them "in the act."

If she's still on the ship, talk to Diana Allers and THEN check your e-mail.

Trivia

 * The area right before the Illusive Man's room, as well as the station name itself (despite the different spellings), may be a reference to The Incredibles, specifically when Mr. Incredible sneaks into Syndrome's lair, only to be shot by cannons shooting tar balls in the main computer room.
 * Some players have reported seeing Kai Leng move his head, rather obviously, just as the scene ends and Shepard is walking away.

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