Priority: Rannoch

The Reaper base controlling the geth has been located, but jamming towers prevent conventional orbital bombardment. Land on Rannoch and use a prototype targeting laser to enable the Normandy to destroy the Reaper base.

Acquisition
To acquire the mission, you must first complete either Rannoch: Admiral Koris or Rannoch: Geth Fighter Squadrons to trigger Admiral Raan informing you of the Reaper Base. To start the mission, orbit Rannoch and select the Reaper base.

If one of the aforementioned missions has not been completed, it will become unavailable once this mission is started.

Walkthrough
This walkthrough assumes that Legion survived the suicide mission. If Legion did not survive its place is filled by a "Geth VI".

Upon arrival at the Reaper base, your squad will be attacked by several geth hostiles: five initial Geth Troopers and a second wave of either Geth Troopers. Eliminate them and proceed down the hill. Once you near the bottom, the large blast door will close and three more Geth Troopers will drop down to impede your progress. Make short work of them. Check near the inside-right of the blast door for a SMG High Caliber Barrel upgrade.

Move to the right of the blast door near the tubes and vault up them. Climb the ladder and eliminate the Geth Trooper waiting at the top. Vault onto the pipe and move forward. Eliminate the two Geth Troopers and Geth Rocket Trooper, and move along the right side to their position. Eliminate the Geth Rocket Trooper on the catwalks in front of you. Pick up an Assault Rifle Piercing Mod and Kassa Fabrication Gauntlets along the way before proceeding to climb up to the next level.

Eliminate the geth patrol below you consisting of two Geth Troopers, and find cover. Four Geth Rocket Troopers will drop in, and once they have been eliminated another four Geth Rocket Troopers will siege your position. Once all hostiles have been eliminated, press forward and through the doors. Salvage the Reaper tech on the right for 10,000 Credits.

Proceed up the ramp and destroy the lone Geth Trooper. A blast shield will expand over the Reaper tech broadcasting the signal. At the junction, go to the left to pick up an SMG Ultralight Materials mod and head back towards to other direction. As you move along the linear path, numerous hostiles will attempt to impede your progress. Fight your way through the geth forces: two Geth Rocket Troopers, eighteen Geth Troopers. Near the start of the walkway, look for an Assault Rifle Stability Damper upgrade on your right. Once all hostiles have been eliminated, Legion will ask if you need assistance. Grab the Geth Plasma Shotgun and salvage the data worth 5,000 Credits on the left of the door.

Move through the door and salvage more data for 10,000 Credits and a MedKit. Proceed through a second door and a nav-point will be marked on your heads-up display. Hostile geth will filter in from the left and the right. Eliminate all hostiles before activating the Door Override. Geth will filter in from the left side; once all hostiles have been eliminated, you will be informed that a manual override is needed and will mark a console on your heads-up display with a nav-point. Proceed to the nav-point and activate the Door Override. Legion will inform you that you cannot double back, but you can use the elevator marked on your heads-up display. Move towards the elevator, but be ready. When the elevator lowers, a Geth Prime will be riding it; eliminate it and the two Geth Rocket Troopers that drop down before boarding the elevator. There's a MedKit on the right of the elevator if you need it.

Ride the elevator up to the next level and be ready for a fight. Three Geth Primes will be standing with their backs to you: one on your left, one on your right, and one straight in front of you. Grab the Geth Spitfire in-front of you - it will come handy against the Geth Primes; in case it runs dry, there will be another one to your left. Also, it is recommended that you take down the enemies' shields first, after which their armor will stand no chance against the Spitfire. After eliminating the three Geth Primes proceed to the nav-point. Shepard will pull the laser designator from his or her back. Designate the target by holding down the trigger and making sure that you stay outside of the blast zone, marked by the blue circle. Note that the target is in front and below you, and not where the objective points. Once designated, the Normandy will proceed with its bombing run. The explosion knocks Shepard and their squad off the upper-platform, and a Reaper begins to climb out of the hole.

Legion tells Shepard that it has acquired transport. Sprint to the geth ship. (If you don't get there in a decent amount of time, the reaper DOES fry you). Shepard will get on top and man the turret while his squad gets inside with Legion piloting. With the ship on the move, target the Reaper off to the left of the ship while Shepard calls in for an orbital strike. (You don't actually have to target the reaper, eventually the vehicle will be far enough and Shepard will yell clear). When the orbital strike hits, the Reaper goes down temporarily while Shepard orders EDI to sync the quarian fleet to the Normandy's targeting systems. Determined to take down the Reaper, Shepard stays back while Legion retreats to a safe distance with Shepard's squad.

Move to the center of the plateau and aim at the Reaper. A set of rotating targeting circles will appear. Begin targeting the Reaper by aiming and holding down the trigger. Once the orbital strike hits, the Reaper will move closer and begin to deploy its beam cannon. Be prepared to dodge to either the left or the right. Once the beam has sliced past you, the circles will appear again. Only during that time can you aim your laser pointer and begin to compute a firing solution. The key to surviving this encounter is to know that, counter-intuitively or not, you don't lose your progress if you have to stop aiming and dodge. Pull up the laser pointer and you'll discover that your firing solution is where you left it and can be resumed. Note also that moving while using the laser pointer causes the target acquisition process to go much slower. The other key is that the Reaper's aim is based on what you are doing when it starts firing.

Dodging while targeting slows progress a lot. The fight isn't hard:


 * The first beam misses you anyway because the reaper is firing up at the flotilla (If you wait around, they start complaining they can't take much more). Stand still and target the reaper. Make sure you aim for the red firing chamber exactly, if you aim a little off, the firing will hold at that distance you missed, making it a very long fight.


 * On the rest of the beams, run to the far right and stand still. Wait for the beam to start firing. As soon as the beam touches ground, turn left and sprint to about half to 3/4 of the ledge. Turn, stand still and fire. Make sure beam is centered, don't move. Keep firing until you see the reaper firing again. Once the beam hits ground, turn right and sprint again. Turn and fire. (if you did this correctly, beam is about your height, maybe a little higher. Feel free to adjust your sprint).

Note that on Casual, you don't have to do this twice. As soon as the beam touches ground, sprint to the side, turn, and fire. The reaper will finish the attack, recharge, and try again. It won't make it. By the time the beam reaches you, you'll finish targeting and it will be hit from orbit. You only have to move once for every beam.


 * Final beam, DON'T MOVE. Just target the eye correctly, stand still and aim. You'll win as long as you don't chicken out and try to dodge or fire off-center. Fire early, keep hitting fire until the gun starts targeting instead of clicking, then hold still.

On insanity, it feels like beam is faster, and being even close to the beam insta-kills you. As a result, you'll need to make wider sprints, corner to corner, giving you less time to fire. Same strategy applies, keep running, stand still, fire for eye. Except run further, corner to corner. As soon as reaper starts firing, sprint, turn, fire.

Alternately, you can start on one side and then strafe to the other side and dive and roll when the beam fires while charging the laser on the target. (Warning: dodge is likely insufficient on Insanity)

Once you complete the targeting procedure, the game will switch to a cutscene of the quarian fleet firing upon the Reaper. While it is disabled, it will speak to Shepard about Harbinger and the inevitability of Shepard's defeat, but will also allude to a higher power orchestrating the entire process.

Depending on what you select as a dialogue, you will get:


 * Paragon (top) choice: Paragon dialogue, about how organics will win. Shepard keeps on organic's side, the reaper brings up the Quarians as an example. A Paragon interrupt will follow. The reaper simply powers off, as if not wanting to talk any more.


 * Renegade (bottom) choice: Renegade dialogue, about taking down the Reapers. A renegade dialogue will take place, followed by a renegade interrupt. Shepard will target the reaper one last time, killing it.


 * Ambivalent choice (help us understand): No interrupt will occur, the Reaper will prattle on about cycles of extinction but offer no real insight, only hints. It then dies of its own accord.

Once this is done, Legion/VI will appear in the frame, starting the aftermath.

Enemies

 * Destroyer
 * Geth Hunter
 * Geth Prime
 * Geth Pyro
 * Geth Rocket Trooper
 * Geth Trooper

Aftermath
The shooting has stopped, but not the yelling. Legion and Tali square off. Legion wants to upload the Reaper code to the geth collective, as it enhances their processing functionality to the point of achieving true intelligence. Tali is naturally fearful of what this might mean for her people, since the geth are currently shooting at them. Shepard must moderate the decision.

Your first choice is to allow the upload, or to let the geth die. By choosing to upload, a second decision branch is opened: allow the upload again, stop Legion, and a Reputation check (Charm: "Rally the fleet" / Threaten: "Warn the Fleet").
 * If you allow the upload twice, Legion will will sacrifice itself to distribute the new programming and the upload will be finalized. The Migrant Fleet will be wiped out by the newly-enhanced geth, resulting in the extinction of the quarian race. Tali will join her people, casting herself off a cliff; there is a Paragon interrupt, but even with it you will not be able to save her.  Shepard will gain the geth as a War Asset, but Tali will no longer be selectable as a squadmate. If Tali did not survive the Suicide Mission in Mass Effect 2, Admiral Shala'Raan vas Tonbay will be present, and will shoot herself.
 * If you choose to stop Legion or choose to let the geth die at the first decision junction, Legion will attack, forcing Shepard and Tali to kill it. The quarian bombardment annihilates the geth in its entirety, resulting in the extinction of the geth collective. Thereafter, the Migrant Fleet joins as a War Asset.
 * Passing the Reputation check is the most difficult, though also the most rewarding, as it leads to the quarians and geth co-operating. The quarians will back down, and both fleets will join as War Assets.  While Legion is forced to dissolve his 1183 runtimes to disseminate true intelligence throughout the geth collective, he achieves true sentience and individuality first, and is able to accept Tali's friendship before passing.

There are so many variables contributing to the Reputation check that, as yet, there is no confirmed understanding of how the game arrives at a pass/fail calculation; even the analysis in the Prima Game official strategy guide has been proven incorrect. As such, readers should take all the following with a grain of salt, and feel free to contribute their own observations to this mission's Talk page where the fandom is continuing to refine its understanding of this pivotal moment. But as to our current analysis, it is thus:

First, there are factors which seem to be purely pass/fail; missing even one of them flunks the Reputation check:
 * Shepard must have at least four bars of Reputation.
 * Tali and Legion must both be present.
 * The player must have imported a save from Mass Effect 2.
 * You must save Admiral Koris

Additionally, there are more factors which build up Shepard's trustworthiness to both parties; if Shepard has not done enough of them, s/he will not have the groundswell of goodwill and be unable to moderate a cease-fire. They are:
 * Destroying or rewriting the heretic geth in Legion: A House Divided. In shipboard conversation, you can ask Legion about the repercussions of your decision, and it will mention either that the heretics were strongly in favor of allying with the "Old Machines," or that their absence made the consensus more difficult to achieve.
 * Preventing Tali's exile in Tali: Treason. If you did not import a save, Tali was "canonically" exiled and will not have be able to support Shepard with an Admiral's authority.
 * Brokering a peace in the Tali/Legion loyalty argument. It is not currently known what Shepard's "canon" decision on the loyalty argument is (if any) if you did not import a save, nor if it impacts dialogue here.
 * Completing Rannoch: Admiral Koris.
 * Actually saving Admiral Zaal'Koris vas Qwib Qwib, instead of allowing him to sacrifice himself for the sake of his crew. He will immediately back up Shepard during the Reputation-check dialogue, which he would obviously be unable to do if dead.
 * Completing Rannoch: Geth Fighter Squadrons. This may be a pass/fail factor, as players have reported being unable to access the compromise without it, despite having done much or all of the other activities.  Legion references it during the conversation, asking the question that started the Morning War: "Does this unit have a soul?"

If resolved with co-existence, talk to Tali afterwards onboard the Normandy will reveal that the Geth are providing lots of assistance in helping the Quarians resettling onto Rannoch:
 * Setting up housings and settlements in far more efficient manner;
 * To help kick-start Quarian immune system, volunteers have their suits loaded with Geth programmes that function in mimicking mild virus attacks; in time, this would help to boost Quarian's natural immune systems so they could live normally without the environment suit; though Tali admits she can't see the Quarian actually ditch the suit altogether, as it has become part of their culture. If Shepard and Tali are in a romance, they flirt briefly, with Shepard teasing Tali about allowing geth inside her suit and Tali assuring Shepard that the only one getting into her suit will be him.