User with this ability either is or can transform into a Vampire, a being who subsists by feeding on the life essence of living creatures (often in the form of blood), regardless of whether the vampire is undead or a living person/being. While all vampires need some form of life-essence, the quality and quantity vary greatly: from daily to rarely, from needing lethal amounts to barely notable, from sentient blood freshly drained to rare steak. Mentally vampires vary from perfectly normal persons with unusual dietary requirements to predatory, calculating beings, to hunger-driven blood-junkies.