Priority: Horizon

Cerberus may be using the Sanctuary facility on Horizon as a cover for secret activity. Land on Horizon to search for clues about their operations.

Acquisition
Automatically available after Priority: Thessia. Notwithstanding the preparation opportunities described below, it is not necessary to help Miranda nor even for her to be alive in order to acquire and complete this mission.

Preparation
If Miranda Lawson survived the Suicide Mission, she will be pursuing her sister Oriana, who has disappeared inexplicably. Miranda suspects the hand of their father, Henry Lawson. Eventually, she tracks Oriana to Sanctuary. If Miranda is not alive, you will receive an e-mail from Oriana herself after the events on Thessia, in which Oriana claims to be in trouble at Sanctuary.

Miranda will also e-mail you for up to three communications over the course of the game, and action or inaction on these will directly influence the aftermath:
 * After completing Priority: Palaven and then one other mission of your choosing, she will contact you to arrange a meeting at Normandy Dock: Bay D24.
 * After completing Priority: The Citadel II and then one other mission of your choosing, she will invite you to the Spectre office of the Citadel Embassies for a vidcomm chat.
 * After Priority: Geth Dreadnought, she will arrange an in-person meeting at the apartments of the Presidium Commons. If you romanced her in Mass Effect 2, you may choose to "lock in" the romance at this time, leading to a dalliance with her.

Squad Prep
You will be facing a wide variety of enemies on this mission, but the toughest battles involve large elite Reaper forces. Plan your squad to be ready for armor and barriers--consider Power Combos that detonate fire or biotic explosions in particular.

Walkthrough
When you are dropped off you see the entrance area to the facility has been close to completely destroyed. Walk towards it and a Cerberus shuttle will crash above the entrance, checked upon by a Harvester that apparently shot it down. You can't fight it, it flies off after seeing the wreckage. Shepard remarks that it's about time Cerberus and the Reapers started fighting each other. A few steps onward and you'll have to deal with two Phantoms that come from the crashed shuttle as well as a Nemesis on the balcony above. There's plenty of debris in the area for cover, so take your time dealing with the ladies. When that's done, to the secluded depression on your right are parts you can salvage for 1,500 credits.

Continue into the facility; there is a medical station by the door. If you paid attention to the eye on the bottom of your screen and pressed the button for navigational assistance, one of your squadmates will point out the Cerberus shuttles seemingly appearing from the tower above and Shepard will say that's where the team is going. But first, if you're not in a hurry, explore the reception area.

Near the windows on the leftmost side, below the conspicuous overhead screens you can examine a terminal with security footage of Sanctuary. In the area apparently used as office space, there are 3 audio logs and a PDA to examine, all pertaining to Sanctuary's refugee processing administration, as well as an inventory log worth 1,500 credits. Clean them out if you want then proceed downstairs.

Sanctuary Reception
To the left by the crashed shuttles are 2 audio logs, left behind by civilians by the sound of them. In the lobby to the right as you came down the stairs, the one with the big glowing SANCTUARY logo on it, there a few dead rachni and Cerberus goons. This should alert you to prepare for war, since just beyond the big wall obstructing the view there are a few Cerberus Troopers around a console to get rid of.

When they're dead, pick up a Pistol Scope mod next to a dead Cerberus soldier in front of the raised desk, then continue through the door labeled POOL MAINTENANCE to the left. A console will have a recording of Miranda (if she's alive) or Oriana (if she isn't), telling you to turn back and that the facility isn't what you think it is. After watching it, salvage the equipment behind you on your left for 1,500 credits and continue through the opening.

You'll come to a large ornamental pool that doubles as a water processing facility as well as a backdoor into Sanctuary's inner facilities. You can't miss it, it has a lot of greenery and flowers in addition to a dead Cerberus trooper floating facedown and a Harvester corpse. Use the water pump on your left to drain the water, head down the ladder. One of your squadmates will examine the strange machinery formerly submerged in the water, pointing out it has Reaper tech. That said, go through the door on the former lake's other side. As you reach the second chamber, salvage the equipment for 1,500 credits and pick up Rosenkov Materials Shoulder Plate on the left of the next door.

Continue on until you get to a room and are told you need to turn on the power; to do so, use the computer in the middle of the room. You will see footage of Cerberus turning the refugees into Husks. In the next room there is yet another console you'll need to use in the corner on the far end, it shows Oriana/Miranda shutting down the power and Kai Leng not too far behind her. Also in this room is a PDA log that explains about the "recycling" being performed. Go down the stairs and open the doors, but be prepared for a lot of husks. In the next room with husks, there are two logs that explain something from Henry Lawson, Miranda's father, about the deception.

Laboratories
The next room is a long hallway stretching to the right, divided lengthwise. The larger of the two spaces, the one up the stairs is more exposed, but the sheltered one, behind the door, is of lower elevation. And both of them are exposed to a ledge on the left side of the length of the hall, from which Marauders will shoot down at you while you face a Banshee and a Ravager, which will spawn once you either open the door or climb the stairs. There is a Barrier Engine in the upper area.

Honestly, your best bet might be to retreat back up the stairs you came from and camp: this will lure the Banshee and Ravager out of position, separating them from the Marauders and denying them their terrain advantage. Once you're done, replenish your ammo and move on to the other side of the room and down the stairs. You'll enter a room where there is a medkit and two more logs from Henry Lawson. You could see another Banshee behind a window, but it can't get to you as it's behind a sealed room, so ignore it and go through the door on your right.

Search the Perimeter
You'll know everything's dead when the objective changes to "Search the Perimeter". There's another console with a Lawson daughter's voice coming from it. Watch her announce she's heading to the tower to disable the communications scrambler. To your left, you can pick up data worth 1,500 credits. There is also a PDA here. Head up the stairs towards a two-door hallway. Prepare to encounter a group of Marauders and Cannibals, supported by four Barrier Engines. Try to deal with these from the doorway or a general distance; entering the hallway leaves the player prone to being flanked. Three of the Barrier Engines, including the one behind you while looking through the two doors, can be taken out from the entrance area.

Once you're done, go to the raised right side of the lab area to pick up the M-12 Locust and a Shotgun Blade Attachment, then to the left side for more logs and a PDA. At the far end and on the left, there's another Lawson Daughter Terminal, containing evidence her father is working for the Illusive Man (in case you hadn't figured it out by now). You then get another clip of the Illusive Man and Henry Lawson talking about their experiments, which are about gaining control of husks. Climb up the ladder Shepard just lowered.

Fortified Ravagers
Up the stairs is another double-hallway fight. Ravagers, Cannibals and Marauders are backed up by two Barrier Engines nestled between the central support struts, one near you and one further up. Move between cover spots, keep your shields up, and keep shooting. Keep in mind that squadmates may often walk straight into Ravager fire, so try to assign them to cover rather than risk them running about. If you have a penetrating ammo mod or a sniper rifle that comes with it built in, you can shoot the nearest and most dangerous Barrier Engine through the glass without coming under Ravager fire, and then followup with some headshots to Cannibals.

You'll know everything's dead when the objective changes to "Search the Perimeter." Head down the hall and you'll enter a room almost identical to the last but without the lasers and bullets flying everywhere. There is a medical station by the door you came in. Salvage the research logs on the left for 3,000 credits. If you play the log on the husk tanks there's mention of a weakness the Reapers have. Go across the walkway to your right through the doors and jump across the gap. There's a power box at the front of the tank-object-thing you're standing on, right there on the floor; shoot the panel off and activate it. Jump off when you're at the other side of the room and go through the door. Time to fight again.

Final Battle
There are two Brutes, and then a Brute and a Banshee, in this room. Keep moving, but be careful how far you advance: the Brute/Banshee combo seems to be a geography trigger, meaning you might fight all of them at once if you run too far up the hallway. Also note there are more places to run and dodge than is obvious at first glance - you are allowed to run on the diagonal glass windows directly to the right of the wrecked platform on which you enter the room, making this effectively a "three hallway" battle. Run around to stay at long range as much as possible while taking these enemies down, since they are all most deadly at close range.

There's a medkit on the left side of the hallway (the side across from where you came in), so rely on that if necessary. After you're done, head to the other side of the hall for more research data worth 4,500 credits, plus a hologram Pod Control that you can activate. Use the "Pod - manual override" option and then head back to the start of the hall towards the empty pod. Inside is the M-99 Saber.

Confront Henry Lawson
Go right, where you'll find an elevator. Save if you want before entering, it's the last time to do so on this mission. Just beyond the final door are ominous sounds of a shootout, however no one dies until you proceed.

It's cutscene time on the other side, with Henry Lawson holding Oriana hostage. If Miranda is present, she'll be at the standoff as well. The choices you made throughout the previous events regarding the Lawsons' story will all boil down to this scene.

Aftermath
There are multiple potential outcomes:
 * If you ask about Henry Lawson's legacy, the camera will focus on the cracked glass behind him.
 * If you can't pass the Reputation check and "just want Oriana", you'll have a Renegade Interrupt option to wound her in the leg, distracting Henry:
 * If Miranda is present, she will slay her father via biotics and Oriana lives.
 * If Miranda is not present, Shepard can kill Henry with another Renegade Interrupt. Otherwise, Shepard will tell Henry to run away, letting him go.
 * If you don't take the Renegade Interrupt and Miranda is not present, Oriana will backpedal towards the glass and throw herself and her father down, which ends in both of them dying. If Miranda is present, she'll kill her father, but her father will also mortally wound her.
 * If you do pass the Reputation check, you'll be able to use a Charm/Intimidate option that gets Henry to release Oriana. If so, you'll get a Renegade option that allows you to kill him. If Miranda's present, she'll take it before you get a chance to.

If Miranda is present, the following conditions will determine whether she dies.
 * If Miranda wasn't warned about Kai Leng, or if you didn't complete her Loyalty Mission, she'll be gravely injured and die at the end. Warning about Kai Leng requires reading the dossier on the assassin before speaking to Miranda over vidcomm, and completing her Loyalty Mission means you must have at least imported a Mass Effect 2 save.
 * If you break off a romance with her during Mass Effect 3 she will also die, regardless of whether you did both her loyalty mission and warned her of Kai Leng. Breaking off a romance during Mass Effect 2, even at the very end before importing the save, will not result in her death. A romanced Miranda will also survive if you have "locked in" another romance before the apartment scene - only an explicit on-camera breakup leads to death.
 * If you deny her access to Alliance resources during your third meeting, or if you never met for the third time on the Citadel, she will die.

Miranda may appear wounded in the cut scene, but can still survive.

Whichever Lawson sister you've been chasing will disable the scrambling tower, allowing her warning to pipe out and guarantee that no one gets near Sanctuary. In the meanwhile, one of your squad members manages to salvage a little data to start you on your way to finding the Illusive Man.

If Miranda was present for the mission, then afterwards an intoxicated Tali will appear in the Port Observation Lounge, having acquired a new respect for Miranda. This will occur whether Miranda lives or dies.

If Miranda's not present, Hackett will use that data to track down Cerberus Headquarters; if she is, she'll confess she planted a tracker on Kai Leng that will take you there. It is also possible for her to tell you about the tracker and then die. Also, if Hackett tracked down the Cerberus HQ, he'll add that the losses will be higher because the Illusive Man knows that they'll attack.


 * The mission Priority: Cerberus Headquarters becomes available.

Enemies

 * Assault Trooper
 * Banshee
 * Brute
 * Cannibal
 * Husk
 * Marauder
 * Nemesis
 * Phantom
 * Ravager
 * Swarmer

Bugs

 * In the room where a banshee is trapped behind some glass, the only exit is a door that occasionally causes you harm when you try to access it. The only known fix so far is to restart the mission. To date, this bug has been confirmed for the PC, Playstation3, and Xbox 360 versions.

Trivia

 * At the conclusion of the mission, Shepard's final line is "I've had enough of this place." This is similar to the line Shepard says at the very end of the mission on Horizon in Mass Effect 2 ("I've had enough of this colony").