hawke + inara (+ simon)
It's very powerful, very stupid magic that uses blood as to fuel it instead of mana. Normally to use it effectively, you need to call up a demon for instruction and hope it doesn't just turn you into a vessel.
Luckily for you, I know some purification spells already and won't have to chat up a demon to use them. Think of it as ... spirit healing's evil twin, in a way; you call on a spirit for either of them, but blood magic is more versatile and more likely to go wrong.