I'm not sure this is the appropriate place for this but here goes:

The fic I'm thinking of was a HP/DF crossover, wherein the White Council goes to war against the Wizarding World and wipes it out. Potter ends up making a deal with Satan or somesuch to go back in time and stop it. The catch is, when he goes back he's possessed by the devil, who is attempting to initiate the End War. The Host is up in arms about it, and sends several agents after him, while Harry is busy trying to get revenge on the White Council.

It's been a while since I read it, and I was hoping someone would recognize the summary.