I also got stuck on this part. Spoilers kinda. I couldn't get the sigil to work (assuming its the one where grove fantasizes about jumping down into the pit.) I did have the idea to lose to one of the slugs, but after defeating the boss, the dialogue for the scene ends abruptly and it seems to be frozen? Can't move or anything.
Edit: Oop, silly me. The dialogue issue's been patched in one of the hotfixes.