Priority: Earth

Acquisition
This mission is acquired immediately after Priority: Cerberus Headquarters.

The dialogue options to start or postpone the mission do not effect Renegade or Paragon scores.

As noted previously, this mission is part of the end-game sequence. By beginning Priority: Cerberus Headquarters, the player forfeits the freedom of the galaxy and can (from that point forward) not fly anywhere except Earth itself.

Preparation
The main thing you'll probably worried about right now is your War Assets. The good news is, you have enough of them; you've had enough since you finished Priority: Tuchanka. However, having more than enough War Assets will result in Earth not taking as much damage over the course of the counter-attack. It is, of course, too late to do anything about that now, but rest assured that you will probably be able to defeat the Reapers. The only remaining question is how decisive a victory you will win.

Squadmates
Your main priority should be damage output. There are a lot of foes, and many of them have tons of shields and hitpoints. Liara and Javik in particular can be a real liability, because they don't do a lot of damage, can't take much of it either, and their powers are rendered all but useless by the mad amounts of barriers flying around. EDI and Kaidan, who are more intended for anti-barrier work, will do better, but not by much. It's Tali, Garrus, James and Ashley who are your best bet. But you're given a chance to re-select squadmates before the final push, so the first half of the mission can be a good opportunity to experiment.

Take Out AA Battery
Hackett and Anderson will walk Shepard through the plan they've put together (which basically amounts to, "Ships shoot Reapers while ground forces try to reach the Conduit"), but like all battle plans, it's tossed out the airlock the moment the Normandy pops through the Relay. Instead of flying straight in, you'll need to stop and take out a Reaper AA battery so that the ground forces can get through. Prepare for the fight of your life; this first one is endless. The screaming of the AA gun doesn't help. Turn down your sound if you want to get through the day without a headache.

After you finally clear out your LZ, you'll get to advance to what seems to be the remains of a house (if the stove is any indication), where you fight off more hostiles and then get to use a M-920 Cain to knock down the AA gun. There should be a second one nearby. Grab it, and then retreat back to the west side of the house: if you stay on the east side where you shot the AA gun from, you will be overrun basically immediately--if the Cain's blast radius doesn't get you as well. From the west side of the house you can use the Cain to clear out the first wave, which consists of a Banshee and some other assorted nonsense. It may seem a waste to use a Cain just to kill one Banshee, but in such close quarters, it's almost impossible to defeat it using conventional arms.

The only problem with retreating to the west side of the house is that when extraction arrives, it's on the east side. Run for it.

Meet with Anderson
You're now in the Forward Operating Base, where Hammer will assemble before making a move on the Conduit. You'll have a chance to talk with however many squadmates you have (it could be as few as three, if you killed Tali and Garrus during the suicide mission and the Virmire Survivor during Priority: The Citadel II, and as many as seven if you got the From Ashes DLC), as well as the current leader of the krogan (Urdnot Wrex / Urdnot Wreav), Primarch Victus, and, via a vidcomm terminal, Cortez and any living Mass Effect 2 squadmates. Enjoy these moments, as they are basically the game's last farewells to your favorite characters.

Note also that there is a one-way door leading up to Anderson, just beyond where Liara is standing. Urdnot Wrex, EDI and Tali are all on the other side of this door, so don't panic if you haven't found them yet. After you speak to Anderson and make a speech to your squad. you're given a final chance to choose two squadmates and adjust weapons. Then it's time to face your destiny.

Approach the Citadel Lift
The first half of the mission is fairly straightforward: advance, kill things, don't die. You'll know you've hit trouble when Shepard opens a door and the game moves to a cutscene, in which ground forces attempt to take on the Reaper Destroyer and, to put it simply, fail. After this cutscene, you'll emerge on an intersection with a Thanix Missile truck in front of you and another one about thirty meters up from you. Get used to this zone: you'll basically spend the rest of the game in it.

The first thing that happens is that your objective changes to "Secure Missile Battery"--after you clear the zone, you need to use them against the Reaper, which requires you to link them to EDI up on the Normandy. She basically tells you that you need to get those missiles prepped and fire them at the Reaper. The second thing that happens is that your objective changes to "Defend Missile Battery," and reinforcements arrive: first a wave of assorted ground troops out of the front, and then, from the left side, Marauders and two Banshees. There is a heavy weapon in the shop to your left, but you might want to save it. Be careful: no matter where you're hiding, you're vulnerable from all directions. This intersection is horribly exposed.

Once you clean up this batch, EDI will announce that the missiles are ready to fire. Considering that there's a second truck of them upstream, guess whether these ones hit. Go on, guess. Yep, they miss, because the Conduit is distorting their aim. You're going to have to bait the Reaper into coming towards you.

You will now face six Brutes, a Harvester and two Banshees, while the Reaper fires its insta-kill laser at you. If you succeed at downing those two Banshees, infantry squads led by Banshees will begin spawning. At this time, the music will start up, as though suggesting that the fight may have gotten a bit intense.

Kite like a champion; the area is fairly open. The Harvester can be dealt with by hiding in a shop and using a window strut to block its line of sight; plus it has the blue weak point on its belly. (It's actually the easiest thing to kill.) Rely on powers and your squad's firepower as much as possible; this battle is easily long enough to drain your ammo reserves, and there are no enemies that drop ammo until the music starts. The Reaper's laser might actually damage some enemies for you, so if you can exploit that, more power to you. The Banshees will likely give you fits, but you don't necessarily have to kill them: if the Reaper has advanced sufficiently, you can dash up to the missiles and end the battle right then and there. What makes this fight challenging is that you have no margin for error. Each enemy has so much striking power that you might as well have just one hit point. Your squadmates will prove this by getting pounded flat about a dozen times. Spend your medigel liberally, you won't need it after this.

Ascend
You will now be in sight of the Conduit, but "several Sovereign-class Reapers, including Harbinger," will descend and start vaporizing things.

RUN.

After that, advance some more. You'll have to protect yourself, but you'll be in permanent slow-motion, so it shouldn't be too hard to line up your shots. Besides, you haven't made it this far to let three husks and a Marauder stop you, right?

The Citadel
You'll now be aboard the Citadel. Evidently Anderson is the only other person who made it. Advance as best you can; it'll be kind of obvious where to go. Anderson will speculate that they're once again trying to build a new Reaper, but that's probably academic at this point.

Finally you'll see someone interacting with a control panel. Approaching him will kick off the final conversation-battle of the game, against the Illusive Man.

There will be several decision branches. If you can't pass the Reputation checks, then make sure you take the Renegade interrupt to prevent the Illusive Man from shooting Anderson. If you somehow miss that, make sure you take the second, or the Illusive Man will kill Shepard and you'll end with a Game Over. For once, though, not using the Charm or Intimidate options yields a potentially better ending: without them, the Illusive Man will have one last bit of dialogue as Shepard opens the Citadel's arms, allowing him to see the thing he has always loved best: Earth.

The Crucible attaches itself to the Citadel. Shepard is then given the series' final choice. There are a maximum of three options; however, if the Effective Military Strength of your War Assets is low enough, you will simply be assigned an ending based on your decision about the Collector Base. According to outside sources (IGN and GameFAQs), you need an EMS of 2800 or higher to unlock the third choice. Build your EMS to 5,000 or higher to unlock the best possible ending, but know that this might require use of Mass Effect 3: Galaxy at War activities.

Aftermath
An ending sequence will run, largely similar in its particulars. Some scenes will be different depending on the galaxy's War Readiness: the higher it is, the better shape Earth will be in. The Relays will be destroyed in Destroy and Synthesis, but in Control it is possible they survive, especially as the Citadel is not destroyed like in the other two endings and the Charon scene is different. Certain decisions will cause an additional scene to depict the fate of the Normandy; one known decision is that following Anderson's example will allow Shepard to possibly survive. Choosing destroy or control will give the same sequence. Synthesis leads to a scene with Joker, EDI, your romance partner and the leaves showing synthetic circuits on them. Finally, certain decisions result in a stinger scene in which an old Stargazer tells tales of "The Shepard" to a young boy.

Trivia

 * When Shepard gets up after being knocked down by Harbinger, miniature versions of the trees Shepard has seen in their dreams will appear on the way leading to the conduit.