Spoilers... what
What was it? So many questions.
shareIf you read the books (or at least the first one) you will know a bit more about the creature.
In few words (if you don't mind spoilers), 'they' (yes, there are more) were once people that gave up something they hated about themselves or something that defined them, in some sort of ritual, in exchange for immortality. And in the books we learn that that was more than 10.000 ago.
But it is really much more interesting reading the books than my explanation.