I intend to DM a dark medieval fantasy game using the d20 Call of Cthulhu book and the equipment lists from the System Reference Document.
Checklist for a dark fantasy game:
1. blasphemous gods (CoC has them)
2. hideous monsters (Coc has them)
3. twisted magic items (CoC has them)
4. corrupting magic spells (CoC has them)
Also, I like the lack of non-human PCs and the lack of different classes.