I tried to run servants of the overlord epic eariler today, and the event where the Rakshasa lord spawns Khyber reavers didn't start.
When I first reached the place where the event is supposed to happen, there was nothing there (Rakshasa didn't spawn), and was just stucked by an invisible barrier (I mumbled something about invisible walls in quests!). I then backtracked to see if I missed a way.
When I came back the Rakshasa spawned, but the event didn't start: they never created the fire barrier, and the reavers didn't spawn.
I tried to reproduce it few times both on normal and epic but couldn't manage to (I'm sure you need a few hard to reproduce bug report to keep you busy ).
Edit: bug happened solo on a FvS (so may have something to do with wings)