The March of Ages
Samuel was formerly a normal human, living in the village nearest the Rift. The energy from the Rift warped the land around it, and he was possessed by some sort of dark entity emanating from the Rift. He somehow was able to control the demon, and in so doing became something more than human.
When human, he and Diehnna were lovers. After he turned, he killed everyone in the village but her in his hunger for blood. He continued caring for her, but would not speak with her, essentially condemning her to months of solitude.
He created and trained Ethera initially, but she is able to feed differently.
Xerus indicated to Stangil Drakebane, Serafina and Elrathia King that Idara may have done something to Samuel when Samuel stood up to Risis on Xerus’ behalf. He never saw Samuel leave Idara’s tree, though.