There's a red switch somewhere off to the far right, after you jump down a hole, on the wall. It doesn't stand out at all... You need to flick that first. There's only one way out of that room, a teleport. Use it to get back to the entrance.
Then, at the far top, there's another hole to jump into with a teleport in the top corner and an exit at the bottom. Ignore the teleport and go through the exit. Take a succession of stairs going up and eventually you will find the demon portal thing. If you didn't flick the switch there will be a physical barrier you can't cross.
Then, at the far top, there's another hole to jump into with a teleport in the top corner and an exit at the bottom. Ignore the teleport and go through the exit. Take a succession of stairs going up and eventually you will find the demon portal thing. If you didn't flick the switch there will be a physical barrier you can't cross.