Destroy the Communications Array
End Security Lockdown
The first thing you need to do when you enter the grounds of the fort is end the security lockdown. You’ll be met by a fierce resistance from the Banished enemies, including a number of elite Brutes, and some Jackals and Grunts. As ever, make use of the blast coils in the area to inflict massive AoE (Area of Effect) damage to multiple enemies, and clear the area thoroughly of enemies. The grounds of this fort also has a plasma cannon near the centre of it, so you’ll want to kill the Banished operating it or destroy it completely as soon as possible.
Once you’ve killed enough of the Banished to have a proper look around, use AI Scan to highlight the structure which contains the control terminal. You’ll need to get on the roof of this structure and walk down a ramp inside to reach the terminal. Inside you’ll find a few Banished enemies, but they shouldn’t cause you much of a problem. Once you’ve ended the security lockdown, you’ll trigger a new objective: expose the array power cells.
Expose Array Power Cells
To expose the array power cells, you’ll need to enter the main structure beneath the horn, kill the enemies inside and interact with the terminal there to raise the power cells. The terminal that you need to interact with is protected by numerous enemies, but also two defensive guns which are mounted on the ceiling. If you go to the back of the structure inside, you’ll find a cooling pylon which you can destroy to disable the defensive guns. Alternatively, you can destroy them by shooting them with powerful weapons. Once you’ve cleared the area, interact with the terminal to expose the power cells.
Destroy Exposed Power Cells
Now that the power cells have been exposed, you can climb (using Grappleshot) to the top of the Horn of Abolition and shoot each of the exposed cells. There are four of them in total, but it won’t take long to destroy each one. Reinforcements will arrive at this point, but it’s just more of what you’ve already seen in terms of enemy types. When the power cells are destroyed the outpost objectives will be complete, but there are some collectibles to obtain here, as shown below.
Horn of Abolition Collectibles
Banished Audio Log 1
The first Banished audio log can be found inside the structure where you found the terminal to end the security lockdown.
Banished Audio Log 2
The second Banished audio log can be found at the back of the structure where you interacted with the terminal to expose the power cells.
The Spartan Core can be found in the ruins of a ship outside the main fort.
UNSC Audio Log 1
The first UNSC audio log can be found on a hill to the southeast of the main fort.
UNSC Audio Log 2
The second UNSC audio log can be found on the road to the northeast of the fort.