A True Name is the name of a Fallen that reflects their true nature and identity as a formerly divine being; they were originally granted to them by God to reflect their position in Heaven and were altered when the Fallen were spurned. They are unpronouncable by mortal tongues because they are not words or names in the traditional sense; rather, they are a collection of abstract sounds and concepts, such as the sound of a hundred children laughing at once or the wind whistling over the entrance of a cavern.
True Names are kept as strict secrets to all but a Fallen's most trusted companions, as they can be used against them in many ways. Knowing a Fallen's True Name makes it easier to summon them, follow them, command them, target them as part of a ritual, or unmake them; those who use a Fallen's True Name against them also gain advantages against the Fallen in mental or social situations. A Fallen or mortal with patience can crack a Fallen's True Name by examining their identity and purpose, but the name must be known in its entiretly; a fragment of a True Name is useless.