Yggdrasil's Seed is a kernel born from the immense ash tree Yggdrasil. Created by Proteus in the age of legends, wielding the seed is thought to grant is bearer nominal control over its plane-spanning pathways.
Clutched in his fell claws, in 722 AF the demon prince Pazuzu used the germ to focus his black will across the multiverse, casting down first the Garden of the Gods, followed by various worlds and regions choked in the snarling brambles of the corrupted world tree.