There are 3 teleporters in Barnabus' cave leading to areas you need to explore.
The top one (on a high ledge) leads to a cave with 3 sections (containing the book Igor wants, if you picked him as your necromancer), while the mid level teleporter and ground level one each have a generator that needs to be disabled (to get rid of the shields around the nest and Barnabus). You should clear the top cave before taking on Barnabus, as some people have had trouble with the doors there if they waited until afterwards (opponents could be missing then, but defeating them is what triggers the doors to open).

For the bottom cave, from the walkthrough:
1: leap across a series of falling platforms
2: climb the ladder
3: use the moving platform to access a lever that activates a teleporter
4: Drop down and take the teleporter
5: navigate several more moving platforms, then climb another ladder
6: time your next jump carefully - the next platform also sinks
7: jump from the sinking platform to another moving platform
8: jump up to another teleporter and step inside
9: hop down a series of platforms in the corners of the room
10: flip the lever
11: wait for a moving platform to emerge from by the door, then jump against
the wall and fall onto it. Good kitty.