I want to thank everyone for their ideas. I appreciate the feedback. This is a little outside my expertise as a DM. I'm a big fan of D&D being about heroism and not evil but This was a little different. It was done so naturally that I had to go with the flow and let it happen. the basic thrust of the storyline is this:
The adventurers were all formerly epic level heros in the distant past. World renowned and respected, they made a lot of enemies. In the twilights of thier careers, their enemies came together and in a massive battle defeated them. The ultimate punishment was that they were level drained back to first level, their mighty talismans were destroyed, and they were imprisoned in stasis so they could not experience the afterlife they so richly deserved after a life of heroism. Two thousand years later they were freed by wizard exploring the ruins of their civilization, long since destroyed because they were not there to save it.
So they set off to become heroes again.
one of the characters was an elderly cleric, basically the pope of his church. He awoke stripped of his mightiest powers and enfeebled by age. he struggled to maintain his faith but was too weary to rebuild his shattered church. He proved easy pickings for the temptation of Grazz't. Once fallen, he realized he would swiftly be discovred by the party paladin so he devised a method by which to change the paladins alignment and the paladin soon became an unholy warrior in service to Grazz't as well as imbued with the power of half fiend.
At the same time, the party ranger, last of his race, searched in vain for a method to return his people to life. He eventually succumed to the whispers of the dark god of nature who vowed to bring his people back to life in exchange for his service.
A second cleric/magic user who hated the undead with all his heart found a book that could transform him into a necromancer of great power. Deciding to destroy the book, he tried to destroy it but found he needed to use it--just the one time...
And then a second...
Each person fell to seperate temptations, some placed in the adventure by me and others they invented themselves. And each one fell on their own, in secret, from the rest of the party. You should have seen the gymnastics each did to prevent the others from finding out

It was a thing of beauty. Normally our group is mostly hack and slash but each and every one of them did a great job figuring out what their character would do in the situation and then following through, regardless of the consequences.
I've decided that I will let it out naturally and then freeze the game in order to talk seriously about what the group should do. Hopefully it will go well. Any other advice anyone else has for running an evil campagain that won't dissolve into backstabbing would be great. I tend to be rather freefrom--probably how I got into trouble in the first place.