If you're being attacked by a pesky 2HP ally or just want to deal some extra damage to your opponent, use Vampiric Bite. Not only does it deal 2 damage, but it also gives 2HP to you, just as a Vampire Ally such as Vampire Prince would. Like a weaker Burn Soul, it can potentially win you the game and costs much less at only 1 black and 2 normal energy. Of course, you can use this to remove allies to build up Soulseeker and restore HP.
Strong against: weak allies, endgame situations.
Weak against: Interfere, allies with armor.