Demonma--er--demoman: "Hey, you! Outside my prison!"
Urist McGrouch: "What, demon?! Can't you see I'm on my way to throw insults at the goblins outside that are forcing us to ration the sweet, sweet mead? I've got them today, I spent six hours last night thinking up insults and I'm gon'ta make them cry!"
Demoman: "That's just swell."
Urist McGrouch: "You really think so?"
Demoman: "Yeah, sure. So, uh, when you guys opened up Hell, you intercepted me on my way to yell at them too. I was going to use my power that makes their skin rot off. While yelling at them. Yeah."
Urist McGrouch: "That sounds cool, mister! You sound spiffy. It's a shame your cage is forbidden and I can't let you out!"
Demoman: "Tell you what, sport. I'll give you the ability to make the skin of people you disagree with rot off if you disobey the overseer's direct orders while he's watching you and let me out. I won't even kill anyone. Promise."
Urist McGrouch: "Oh, boy!"
[A PRISONER HAS ESCAPED]
Demonman: "I'll be on my way now. Stay safe, kiddo."
[Demonman vanishes in a cloud of smoke!]
[Urist McGrouch receives power: inflict crippling necrosis. Urist McGrouch is now ecstatic. Urist McGrouch has been charged with treason.]
I like the ideas, gentledwarves. I thought of them as persuasive depending upon personality, like being able to sway dwarves, even ones on direct orders to keep them imprisoned. For example, you might have to swap the guards out monthly(if you even use any) or the demon might make a few worshippers amongst them, causing him to be freed. The demon may or may not take the followers with them and may or may not come back later as an attacking force, bringing the followers as high cultists.