Often referred to simply as "the Shadow" or "Stain" this was the residue of Morgoth's evil that inhered in the very substance of the world and everything in it. The Mordo (Q."Stain, Obscurity") or Morifaire (Q."dark radiance, Dark spirit-shape" ) was stronger in Forodwaith and Forochel than in most regions because it had once been Morgoth's realm. The quest of the Noldor of Evermist was to rid the Northern Waste of this unnatural taint.
The Mordo is not a specific entity or personage but an impersonal power, much similar to the Unlife of the Rolemaster Books.