Descend further into the tunnels and defeat every Fallen there.
- Go forward into the first room of the Atrium, you will find a few Dregs, Vandals, and Shanks waiting for you in there.
- Clear out the first room of the Fallen.
- Look along the left-hand side to find a doorway to enter.
- Inside the next room, drop down and go forward through the door ahead of you on the left.
- There is a Dreg in this room you should deal with.
- Turn to the right and start down the corridor you arrive in.
- There is a Vandals and a few Dregs here for you to deal with en route.
- Go through the archways on the right into the final large chamber.