He took the seed from the Tree of life in the overworld nearly 700 years ago and brought the seed to the underdark (current tree). Drow sorcerer queen knew that the seed had come into her realm. The seed dissapeared and the Sorcerer Queen didn’t get the seed, but Avor and his companions were never heard from again.
Avor fell under a strange spell that kept him from taking the seed from where it needed to go. The seed is now the current tree of life.