A True Name is a piece of the World Formula that defines the existence of a given vampire. They are the fundamental building blocks of vampiric existence, defining a vampire's "life"—both their nature and their fate.1 Altering one's true name will cause both physical and mental changes to that vampire.
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The corruption of a True Name is referred to as having one's True Name "stolen."
The first chapter of The Case Study of Vanitas opens with The Shapeless One warning Noé that he must never let his true name be stolen.