Fell taints (sometimes referred to as ‘living spell-lets’) are strange, aberrant creatures. They kill by inflicting madness, feeding on the thoughts and emotions they steal from their foes. Fell taints are only partially real, and are difficult to hurt with normal weapons; however, some become solid when feeding on a dazed or unconscious victim, and can easily be destroyed in that state.
These creatures were brought to Stormreach by Taryn’aashta, who traded them to the Storm Hammers in return for dragonshards and a secret alliance. The Storm Hammers used them as terror weapons, apparently controlling them with sounds and magic, and encouraged them to breed and morph together.
Originally it was thought that the fell taints were formed from the mists of the Mournland, but this turned out to be false. They are in fact beings from a plane of madness that were somehow brought to Eberron by the Mourning. They have a malign and alien intelligence and have used their collective telepathic powers to control and take over the Storm Hammers for their own ends.