The room you appear in is a storage room full of crates that form lots of corridors in a maze-like way. So the first thing you should do, is explore the WHOLE of this big room, killing all the Dark Bishops as you go. Make sure you thoroughly explored the whole of the room - a lot of crates or even piles of crates can be jumped upon, revealing something new. If you have all the three Planets, you have finished this level and can take the portal back to Heresiarch's Seminary.