In a far distant time, when the multiverse was new formed, the various planes and dimensions were linked and minions from each were free to travel at will between them through gateways that formed where the physical manifestations of the planes intersected each other.
One such gateway existed in a place that shall forever be known as Shadow Mountain where a prime material plane intersected with the Plane of Shadow. It was here that many ancient wars were fought and the gods themselves were almost deposed. Heroes of both light and darkness battled the minions of the shadow realms and their allies until a messenger of the gods, Brodram, stepped forth into the breach sealing both sides and trapping himself for all eternity in the void between worlds.
Such events have passed into myth and legend in the world where lives are measured in brief years but not so in the Realms of Shadow. Their defeat at the hands of mortal servants of petulant deities has festered like a sore, oozing with hatred and the lust for revenge.
While in the world of mortals great heroes continue to battle with allies of the shadow realm, trapped here during those events many millennia past and not subject to mortal law. Malevolent and undying, in defeat they are banished only until their essence reforms.