Unfortunately, that quest is bugged. You should probably save before going in that area and then complete it on a separate save. It also hasn't been fully implemented yet, so you can't truly finish it.
There is also a second way that I got stuck in there that isn't truly stuck. If you are trying to leave there is a rune on the ground behind the blue flames. You have to click or interact with those to leave. This only works if you aren't bugged/glitched out after defeating the boss. If that is the case, you have to go back to an earlier save.