In Mass Effect 3, Commander Shepard hardly had time to deal with the Illusive Man and whatever insane plot he and Cerberus were attempting to carry out. At every turn, Cerberus was there, going against all the hard work Shepard put into place to foster alliances and build a stronger galactic force to face the Reapers. Cerberus had almost become an even greater threat to humanity and the rest of the galaxy than the Reapers, and they needed to be stopped.
Admiral Hackett believed that Cerberus might be using a facility called Sanctuary on Horizon as a front for their nefarious plans, but not even Hackett could have imagined what Shepard and their squad found there. To make matters worse, Shepard's old ally, Miranda Lawson, was there trying to find her sister and expose Cerberus and her father. In a race against time, and Cerberus assassin Kai Leng, Shepard battled through a facility packed with Reapers and hideous Cerberus creations to take out Henry Lawson and get to the bottom of the Illusive Man's agenda.
Mass Effect 3 Mission: Sanctuary Arrival
In the landing zone, explore the entryway and watch a Cerberus shuttle crash over the facility before a Harvester swoops in to make sure there were no survivors. Advancing closer, two Phantoms and a Nemesis emerge from the shuttle crash. If possible, use Shepard's equipped sniper rifle to take out the phantoms from a distance. Once they are take care of, grab the salvage worth 1500 credits and head inside the facility to pick up a med-kit. A short cutscene shows a Cerberus shuttle flying overhead toward the tower, which becomes the primary destination. But first, explore the Sanctuary reception area.
Among the desks, there is a PDA and three audio logs that shed more light on Sanctuary's processing protocol. View the inventory log for 1500 credits, then head down the stairs to the left. There are two crashed shuttles in this area, each of them containing audio logs from civilians who traveled to Sanctuary in search of refuge. Back inside, take note of the dead Cerberus troopers and Rachni Ravagers on the ground and advance carefully to trigger the next battle.
Take out the Cerberus Troopers and grab the Pistol Scope in front of the desk, then head left and toward the opening labeled "Pool Maintenance." Before heading into the entryway, there's a console. Access it to discover a warning from Miranda Lawson (note: if Miranda did not survive the Suicide Mission in ME2, this will be her sister, Oriana, instead), directing people to leave because Sanctuary is not what it seems. After watching the warning, access the wall safe for 1500 credits and then proceed through the entryway on the left.
Approaching the pool, there is a control panel that drains the water out and provides a backdoor into the Cerberus facility. Head down the ladder and move forward as one of Shepard's squadmates points out that the previously submerged tech is Reaper Tech. Go through the doors and pass into the second room to find the Rosenkov Materials Shoulder Plate and a piece of salvage worth 1500 credits. Proceed into the next area and head to the console to turn on the power. With the power restored, view the video footage of Cerberus turning Sanctuary guests into husks and notice the surviving husks just beyond the glass.
In the next room is another video of Miranda, and just behind her Kai Leng can be spotted skulking in her wake. There is also a PDA log that offers more information on the procedure Cerberus has been experimenting with. After accessing the PDA, head into the next area for two more logs from Henry Lawson detailing the deception luring people into Sanctuary, his own experiments and work for the Illusive Man.
Mass Effect 3 Mission: The Labs
Those husks Shepard saw will come pouring through the doors as soon as they are opened and the squad begins advancing into the laboratory. After taking them all out, proceed carefully into the labs, which consist of one long hallway divided into two sections. There are multiple enemies in this area, including Marauders, a Banshee and a Ravager. Battle through them and then head to the other side of the room to the staircase. Inside this area is a med-kit and two more logs from Henry Lawson, one of them detailing information about Paul Grayson. Head through the door on the right.
It's time to search the perimeter, which leads to a console with Miranda/Oriana updating her objective, noting that she's heading to the tower to disable the communications scrambler. On the left is a data pickup worth 1500 credits and a PDA. At the top of the next set of stairs are more enemies, Marauders and Cannibals, and multiple barrier engines. Take them out from the doorway to avoid being flanked then go to the right side to pick up the M-12 Locust and the Shotgun Blade Attachment. There is another video log of Henry Lawson and the Illusive Man, revealing their intention to take control of the husks.
Lower the ladder and climb up for another fight, this time with Ravagers and Cannibals. Once they are all dead, the objective shifts to "Search the Perimeter" again, so enter the lab area. There is a medical station near the door and some research logs on the left that can be salvaged for 3000 credits. There is a log on one of the husk tanks that mentions a Reaper weakness. Jump across the gap in the walkway and prepare shoot the tram panel to activate its movement to the next area.
Mass Effect 3 Mission: Find Henry Lawson
When the movement stops and Shepard reaches the final area, there are two Brutes and a Banshee. Stay in cover and try not to advance too quickly to avoid confronting all three at once. Once they've been taken out, grab the med-kit on the left side of the hall, then head down the hallway. There is some research data worth 4500 credits and a hologram pod control switch. Activate the switch, then head back to the empty pod to pick up the M-99 Saber.
To the right is an elevator that will take Shepard up to the tower. Upon arrival are the sounds of gunfire. Rush in to find Henry Lawson holding Oriana at gunpoint. At this point, there are a lot of options dependent upon events leading up to the encounter. If Shepard has a high enough Renegade/Paragon score, they can convince Lawson to let Oriana go. There is a Renegade interrupt that will allow Shepard to kill him, or they can ignore it and offer him the opportunity to run for his life. If the second action is taken and Miranda is present, she will blast her father with biotics, sending him plummeting to his death, but not before mortally wounding Miranda.
There are conditions as to whether or not Miranda survives the encounter. If Shepard did not complete her loyalty mission in ME2, or they failed to warn her about Kai Leng before she went to Sanctuary, she will die. If Shepard did not grant her access to Alliance resources during their meeting in the Citadel apartment, or they failed to meet with her, she will die. If she was romanced by Shepard and they did an on-screen breakup with her in ME3, she will die. But, it does not count if they ended their relationship in the second game or locked into another romance before meeting with her on the Citadel.
If Miranda is not present, and Shepard doesn't have a high enough reputation to charm or intimidate Henry Lawson, they can shoot Oriana in the leg as a distraction. If the Renegade actions aren't taken to kill her father, Oriana will drag him backward and they both plummet to their death. Whichever sister is there disables the communications scrambler. If it was Miranda and she survived, she placed a tracker on Kai Leng that will help Shepard locate the Illusive Man's base of operations. If Miranda is absent, Admiral Hackett uses the data found at Sanctuary to track down the Illusive Man instead.