[Disclaimer] I am not obsessed with arch fiends or Diablo II, this series of threads is for a campaign world in progress. [/Disclaimer]

I've got the Diablo II: - to hell and back - book and it has a lot of interesting new monsters. However, the Prime Evils: Diablo and Mephisto, while they do have stats, for some reason don't have challenge ratings.

Does anybody know a good CR for these two?