Hooks for evil characters?

The key to making campaigns for evil characters is you have to cater to their self-interest. Most of the time, this means giving them a mission with the promise of wealth or power on the other end, but, while that works fine for adventures, it can get stale over a whole campaign. After a while, the players start to realize that there are lots of ways to get money or rank that don't involve wandering ancient crypts or fighting dangerous monsters and the whole thing loses it's sense of urgency. What I'd suggest is making the item they're searching for be so valuable and so important to them that they have no choice but to try to claim it. With that in mind, here's a campaign seed that I think would work quite well for an evil party.

Campaign Seed:
Soul Searching: The Demon Lord Grazz't is fond of claiming the souls of those foolish enough to approach him, and the party just happens to be among that number. He lured them to his domain through a variety of means, but the end result was the same for all of them -- their souls were flayed from their bodies as they writhed in pain. Now Grazz't has given them an ultimatum. They must serve him as long as he pleases, or their immortal souls will feel his eternal wrath.

The basic idea is that party will start out in Grazz't's waiting room about to get their souls sucked out. Once that happens, they will begin embarking on clandestine missions for Grazz't (or any Fiend lord you choose to replace him with), while at the same time, they will be trying to find a way to get their souls back from him so that they can get on with their lives. Grazz't's eyes and ears are everywhere, so the best place for the PCs to look for information and assistance would be Sigil, where Grazz't can't get to them. This will, naturally, lead them to a certain Marauder of our acquaintance. Where you choose to go from there is up to you.

I think forcing evil characters into action is a very good way to get them started. Just as in Ducklucks story. Tragidy and unsuscessfull struggle are also a good themes for meanies (as with the Dark Lords of Ravenloft), as opposed to simple greed.

Since all your chars belong to a Faction, a mission is always a plausible beginning. Receiving some precious item or information. The PCs could be ordered independently and only got together by an unpredictable incident, now somehow 'trapped' to work together but ultimately opposing eachother. Dangerous Dungeons or broken/closed portals might serve as such a trap.

*edit: Just stating the obvious: The cutters ended up in the Blood War! Suits them right, evil and mean and barmy as they are. Motives are easy: Doom for the Doomguard, Fate for the Fated and Dust for the Dustman!