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.

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. You'll get up to three communications from her over the course of the game: an in-person meeting at Normandy Dock: Bay D24, a vidcomm conversation at the SPECTRE office of the Citadel Embassies, and an in-person meeting at the apartments of the Presidium Commons.

The vidcom chat will be available after Priority: The Citadel II and your first meeting with Kai Leng. Anderson will e-mail you Leng's dossier at this time. If you read it, you can mention his presence to Miranda during this conversation. If you don't, she'll die during this Sanctuary mission. Use this knowledge as you see fit.

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.

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 near the exit and a Harvester will fly over you. It's about time Cerberus took a hit from them. You'll have to deal with a few Phantoms that come from the crashed shuttle as well as a Sniper on the balcony above. To your right are parts you can salvage for 1,500 credits. Continue into the facility, there is a medkit by the door, explore the reception area, listen to logs, pick up an inventory log worth 1,500 credits and proceed downstairs. There will be a few Cerberus Troopers around a console to get rid of. Pick up a Pistol Scope mod next to a dead Cerberus soldier, then continue through the door 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. Salvage the equipment on your right for 1,500 credits and continue through the opening.

You'll come to a large ornamental pond. Use the console on your left to drain the water and head down the ladders and go through the door. As you reach the second chamber, salvage an 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. Go down the stairs and open the doors, but be prepared for a lot of husks.

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, you know, face a Banshee and a Ravager, which will spawn once you open the door. Honestly, your best bet might be to retreat back up the stairs you came from: this will lure the Banshee and Ravager out of position, separating them from the Marauders. 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 medikit and you could see another Banshee, but it can't get to you, so ignore it and go through the door on your right.

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. Head up the stairs towards a two-door hallway. Prepare to encounter a group of Marauders and Cannibals, supported by two Barrier Engines. Try to deal with these from the doorway or a general distance; entering the hallway leaves the player prone to being flanked.

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. 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.

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. A bug currently exists that leaves a squadmate stuck on the ladder, so be prepared. 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.

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 medkit by the door you came in. Salvage the research logs 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.

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. There's a medkit on the right 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 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.

Finally, go down to the far end of the hall where you can salvage 4500 credits of research data. Then go right where you'll find an elevator. On the other side, you'll find a cutscene, with Henry Lawson holding Oriana hostage; if Miranda is present, she'll be at the standoff as well; if she wasn't warned about Kai Leng, or if you didn't complete Miranda: The Prodigal, she'll be gravely injured. As always, there are multiple potential outcomes. If Miranda is wounded here (you didn't warn her about Kai Leng) (you did warn her but she was disloyal to begin with), she will then die of her injuries. (Note: if Miranda is present, she may appear wounded in the cut scene, but can still survive.)
 * If you ask about his 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. If you don't take the Interrupt, Oriana will backpedal towards the glass and throw herself and her father down, which ends in both of them dying. If you take the Interrupt:
 * If Miranda is present, she will slay him 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 do pass the Reputation check, you'll be able to use a Charm/Threaten option that gets Henry to release Oriana. If so, you'll get a Renegade option that allows you to off him.  If Miranda's present, she'll take it before you get a chance to.

Whichever Lawson sister you've been chasing will disables 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'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. 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.

Enemies

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

Aftermath
You will gain 15,000 credits as well as the Priority: Cerberus Headquarters mission. If Miranda lives, she will also join up as a War Asset (25). Finally, Traynor will suggest you chat with Tali after this mission.

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").

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 and Xbox 360 versions.