Orb of Unity and Night of the Vampires, making all your allies both vampires and extremely strong. With Orb of Hostility in a black/red aggro deck, he'll last fewer turns but will give you more HP. In a black/blue deck, buff him with Develop on your bench and play him during your [[phases|Defense phase[[ to restore a lot of HP when defending. Be careful, though--vampires are useful, but not very strong.
Strong against: weak allies, helps when you have low HP.
Weak against: strong allies and direct damage actions.