As ColonelHardisson already pointed out, CoC spells cost Ability points to cast. A spell might cost you two points of Int or Wis (leaving your thought muddled), or Str or Con (leaving you tired and weak), or whatever.If you would care to elaborate on CoC's magic system I might be better able to evaluate their compatability.
Also, CoC introduces the concept of Sanity, and spells generally cost Sanity to learn and cast. That part's not very Middle Earth.