Mourel Duskwalker was a Star elf wizard who was corrupt and hungry for power. From his citadel of glass Tir'in'tiral in the demi-plane of Sildëyuir, Mourel marshalled evil forces to conquer the Star elves but was eventually defeated. He was interred in Tir'in'tiral and it, plus all of the towers connecting it were cut off from Sildëyuir to become a separate Demi-Plane known as the Night Realm and eventually relegated tales of "The Duskwalker" to stories for frightening children.
Unbeknownst to the mages who cut it off, a portal that conected the Night Realm to the Yuirwood still existed, though was inactive. Also, Mourel's apprentice Kyjal Stardancer, who was the only survivor of Mourel's defeat, had managed to extract the brain of his master and had placed it a jar of embalming fluid which was then placed in the chest cavity of a Girallon zombie.
Unfortunately for Tolg'byri, his intrusion upon the Night Realm drew the attention of Mourel and the animated corpses also interred in the mausoleum that had been connected to Tir'in'tiral. These, along with Mourel's magical experiments and the undead remnants of his army began fighting with Tolg'byri's troops. Mourel also used the opportunity of his renewed link to the Material Plane to kidnap people during the night when the portal was active and also managed to steal the Grail of Shargrailar from a merchant who had camped near the portal.
During the fighting, adventurers found their way into the Night Realm and put an end to the fighting by killing the leaders of both forces.