Pelagius's Storm Thrall

Pelagius's Storm Thrall is an essential storm atronach that can be found in The Mind of a Madman during the Mind of Madness quest; specifically during the "Pelagius' Paranoia" section. During this quest, two atronachs will be in an eternal fight in a sunken arena. Using the Wabbajack on the one assigned to you will cause it to change elements, possibly becoming this thrall.