Reactivate the repaired security system using the terminal on the catwalk.
- Go through the now open door behind you.
- Run across the walkway in front of you, jumping the gap and going into the Guard Post.
- Go to the left-hand hatch and onto the next walkway.
- Cayde-6 will be supporting you with Manually Controlled Turrets around the area.
- Fight on across the walkways, fighting through the Hive.
- Go through the short hallway and through the next hatch.
- Just beyond that, you will encounter another group of Hive.
- Toward the end of this walkway, you will encounter Cabal.
- Backing them up is Kaaron, Blood Infernal
- Defeat them and go into the room they were guarding.
- Inside, go over to the console at the far end of the room.
- Interact with it to reactivate the security system of the Prison of Elders.