Avassh is one of the 6 known Daelkyr that are currently imprisoned in Khyber. It isn't known where Avassh's lair is, but it has supposedly spread its roots across Eberron.
Most daelkyr follow a theme, and the abominations that they create reflects this. Avassh's obsession is the cycle of growth and decay, and this is most often expressed in the form of plant-life and fungi. During the Xoriat Incursion, Avassh was active in the region now known as Droaam and the Eldeen Reaches, and attempts to combat it by the Dhakaani are what led to the barrens becoming barren.
Avassh didn't corrupt animal life in the same way that Dyrrn did, rather it created or moulded new plants and fungi, some of which interacted with living creatures and transformed them, as with the myconids. Other creations of Avassh include blights, gaa'avassh (treants), various forms of hostile fungi, diseases, and more bizarre creations such as the Trees of Knowledge and the Bone Orchards.