so far I've been thinking disease but that's not decay.
this is my preliminary at the moment
level 1- bane (appearance would be a pain in the bones that seem to reflect arthritis or some such)
level 2- summon swarm
level 3- contagion
level 4- enervation
level 5- nightmare
level 6-
level 7-
level 8- horrid wilting
level 9-
It still doesn't quite grasp me. Horrid wilting is a definite.
Don't even have a granted power, I was thinking like a decaying aura that damages everything living and non living. imagine arrows that don't have arrow heads cause they decayed off the arrow, etc.