Revenance

Revenance is what traditional 'blood-sucking vampires' would entail.
They feed off the blood of others, and hunger for it.
Do they get any benefits for it? Yeah, Sure.
Revenants have increased regenerative capabilities. How powerful? Eh... lets say that they can recover from physical ailments quite quickly. However it comes at the cost of hunger.
So the less blood they have in their stores, the less effective their regenerative capabilites.
Are there detriments to this? Yeah, you cant eat normal food anymore, blood only. Nerd.