I'm in the Planer Jail and I have gotten to the part where I need to destroy the orb to set the thrulls free, but I don't know how. I have a feeling that I'm going to feel like a moron when I find out.

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Worry not for its is indeed a simple answer to your problem. When you get the little green orb from the dwarf guy, take it around back the way you came, heading to the room you first started in. Just as you pass two of those weird pink portals you'll see a buzzing thingamajig (for lack of better word) and when you put the cursor over the center of it you get the arrow turning icon. Click on it and your character will drop the orb in the tube and the thrulls will start to attack the cambion and his minions. From there you're on your own, but I'd summon a few monsters and send em in first to wear the warden out. Best of luck to you! Hope this helps any.