Webs of the Endangerment
The Vale Perilous has quite a few magical monsters in it, including Griffons, Manticores, Unicorns, Harpies, Nightmares, Bargheists, Goblins and Gaunts.
No one in the group is certain what a gaunt is, but that’s okay, because whoever put together the Oracle probably didn’t know what one was either. It just sounds cool.
Currently, we speculate that a gaunt is some sort of flying undead humanoid. Presumably, a guant is pretty darn thin.
By some unknown process, fairy creatures such as these can be bound to a human heart, giving that person virtues or vices which, one would assume, that person never before possessed (or, at least, possessed in such a miserably scant quality that it isn’t worth mentioning.)