At the moment I'm thinking of having a character that has two Personaes annoy the party. A 10th Level paladin in one of his personaes, perfectly good and orderly, a nice chap who lends his healing for free and helps the PCs a good deal, one odd day he asks them to return the favor, since they know the guy from a few encounters, they probably wont mind with detecting either thoughts or evil, he asks them to bring back a powerful evil item for it's destruction, they'll find the item guarded by a bunch of good critters who doesn't want random PCs to dig it up.
Here's the catch though, the goody two-shoes paladin has an evil personae as well, actually it's his main personae, the good guy just takes over a few too many times, the evil personae is a Thrall of Demogorgon, who are wack-jobs, and although his two personaes started out both being chaotic evil, he picked up a helm of opposite alignment in one of the alternatives, and is now retrained into a paladin. The bad side is probably well that he has a good side messing with his plans, but he uses it for his own good anyway or attempts to.
I'll give the PCs a few heads up though, like wanted posters in several towns featuring their friend the paladin and similar stuff. And he is often covered in blood when they meet him, trying to help dying victims that he just attempted to slice up a few minutes ago. How would you think that'd work?