Priority: Horizon is the final mission of the second Mass Effect 3 act.
The events of this mission, take place on Horizon, where you must investigate a science facility controlled by Cerberus.
The following Mass Effect 3 Walkthrough Act 2 Priority: Horizon explains how to complete Priority: Horizon, where to find all weapon mods, armor upgrades and salvageable parts.
The game guide also explains how to finish all objectives while playing this specific Mass Effect 3 mission.
“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.”
To begin Priority: Horizon, go to the Combat Information Center on Normandy and open the Galaxy Map. North-east from Serpent Nebula is the Shadow Sea. Travel to that location and enter Horizons orbit. Land on Horizon to start the mission.
During the introductory cutscene, EDI will offer additional information regarding the Sanctuary, and you will hear Oriana’s warnings to stay away from the Sanctuary.
You will land on a balcony and your first objective is to enter the Sanctuary. Follow the stairs in front of you and soon you will see a Cerberus shuttle crashing in the same area.
Kill the Cerberus Snipers (Phantoms) first, and then take care of the Nemesis.
Advance toward the entrance and stay near the right side of the area, because there is a small ramp which will take you to hidden area (to the right). Search the left side for some Parts you can salvage for 1.500 Credits.
Enter the Sanctuary and turn right to find a Medical Station, near the door you used to get inside. Next, you have to explore the Sanctuary, and reach the other side where you will find the stairs that allow you to descend. In the office area, there are many Pads you can examine, but only one of them will offer credits.
You will find it near the glass wall. Look for a corner desk (not the one next to the stairs). Once you get the credits, follow the stairs down, and turn left, to find additional logs (on the ground, next to the shuttles).
Now explore the next area, and you will see two Cerberus Troopers on a higher platform. Take them out, then search the corpse on the same platform for the Pistol Scope.
Exit through the door on the left and you will enter a small chamber. Turn right and salvage the equipment on the wall, then activate the console in the same room. Exit to the courtyard, activate the Water Pump, and head down using the ladder.
Enter the Sanctuary and cross the first room. In the second room, on the right wall you will find some Equipment that can be salvaged. Near the next door, to the left, you will find the Rosenkov Materials Shoulder Guard.
Open the next door and activate the console in the middle of the chamber to trigger a cutscene and restore power. Advance through the green panel door, and in the next room, check the desks near the entrance for a PDA.
Activate the console and follow the stairs down, killing all Husks you encounter. Next, all you have to do is to push towards the Tower. On your way you will encounter Banshees, Ravagers, Reapers and Marauders.
Make sure you stay in cover and advance until you reach the door which allows you to enter the Tower (the objective is clearly marked on your HUD). In the same room (where the door is located), on a desk, you should see a Med Kit. Pick it up before opening the door.
In the next chamber activate the console on the right side of the first floor, to find out what happens to the Husks. Near the console is a desk, and on it, is a PDA. Now head up and clear the second and third floors.
At the end of the second floor, you will find a console (left side). On the third floor you will find the M-12 Locust and the Shotgun Blade Attachment, on two different desks in the middle of the area. After you activate the console use the ladder near it to advance.
Now, clear the area and secure it, by killing all Cannibals, Ravagers and Marauders. Your goal is to reach the door located on the other side. When you do, look near it for a Medical Station (right) and some Research Data (left). Advance and jump over the first gap to reach the central platform, then shoot the control box to the right, to activate the lift.
Next, go through the green-panel door, and be very careful when you enter the next chamber. You will notice that it was destroyed, and the mayhem was created by several Brutes and a Banshee. Eventually, they will notice your presence and they will attack you. Take them out before they reach your position.
Kill the Brutes first and then the Banshee. After you secure the area, check the upper left side of the room for a Med Kit. Advance towards the Tower and to the left, before reaching the door you will find some Research Data.
Open the next green-panel door to advance. Use the elevator to trigger the final cutscene and to complete the mission.
When you return to Normandy, Priority: Cerberus Headquarters will be unlocked, after you talk to Hackett.