None Of This Was Supposed To Happen
Venethan didn't breathe a word about seeing the demon king to anyone in Zarath's faction the entire time he was there and continued with their magical research as if his head wasn't reeling. Everything he had ever wanted was SO CLOSE to coming to fruition but he had to be patient.
Those eight months were torturously long. Being the powerless fairy king had never been more annoying but eventually he was able to get the spell he needed along with the instructions on how to make it work.
This would be difficult. Unfortunately, taking blood wasn't all he had to do.
That would be sufficient to make himself immortal but not to bind the sacred tree to him instead of Azariel. He would need her wings for that. Much of fairies' magic was stored in them.