The nicest thing about the "Sneaky Powerful Servitor" idea is that when the party figures it out and trounces the bad guy, the demon either dies or gets away...unlike the "Petty Villain with a Powerful Tool" concept, where after trouncing the bad guy, the party might actually end up with that overpowered item and start using it against
their enemies.
I mean, sure, you can make the tool something that isn't usable by anyone else (alignment restrictions, a weird innate ability granted by some other agency, having something pop up and automagically take it away before it takes over the campaign, "looks like it just ran out of charges," simple fiat, etc.), but I always feel a little slimy about doing something like that. Bad experiences with some bad GMs, I guess. But demons, man, they don't make that kind of problem and they're nifty recurring enemies.
Another age-old concept is the weenie who doesn't really know how the big gun works; he found it (or it found HIM), and now he's causing mayhem and destruction and terror more or less by accident. He can't get rid of it, no matter how much he wants to. He can't stop it, because he doesn't really know how he got it started in the first place. While he menaces the party and oodles of NPCs with it, he's also a menace to himself, and the whole thing ends more or less catastrophically. Or with a more competent baddie steering him towards particular targets, maybe.
--
and just imagine how much grief a dim-witted but evil guy could cause with three
wishes
ryan
