Kill the Tainted Black Bears, Tainted Nightstalkers, and Tainted Screechers in the Tainted Forest, and try to assemble their parts into a Demoniac Vessel.
We need a way to get Wormthorn's attention. Something to break him out of his trance. Something that's guaranteed to work.
The few types of wildlife that managed to make a foothold in the Tainted Forest are, without a doubt, affected by the land's latent energies. Your task is to collect their body parts and find a way to combine them into a vessel. If Wormthorn's trance is druidic, or even if it is demonic, a large enough vessel of tainted animal flesh should be able to break it.