Nyrdru was the High Priest of the Galthon, some kind of evil religion or subjugated people. He was mentioned in a stand-alone excerpt in the first edition of the Shadow World Master Atlas. The story was from a few centuries after the end of the Wars of Dominion, when the Loremasters were trying to clean up the mess of all the demonic powers and other forces of darkness still roaming freely. The "Galthon" were unexplained, but the story was relayed by someone from Eidolon, which was on the continent of Emer. It was supposed to be an illustration of what happens to people who lose themselves to the Unlife, or what happens when you try to claim a soul back from it.
Nyrdru was being hunted down by the Loremasters, and had taken refuge in the unholy "Temple of Night." There were no longer any demonic servants to protect him, so he was trapped against the pursuers. His "once-handsome face" was "now the snarling visage of animal rage and hatred", and he cast what was implied to be a Nether bolt at the group. The Loremaster Kedrik had anticipated the attack, and blocked it with a scintillating energy shield.
It was at that point the Loremaster summoned a Spell of Absolution in the Iruaric language, which provoked a shriek that "no human should be capable of" which "could only have been the utterance of the Unlife." The author described it as a shriek "whose origins were beyond the Wall of Darkness", which was the Essence barrier separating the hemispheres of Shadow World, enforced as a consequence of the Eyes of Utha. It was assumed to be a realm of darkness by those who had never been there, possibly a "negative universe" of Unlife, which housed surviving followers of Kadaena. Thus, the story most likely took place on the I.C.E. Age continent of Gaalt, which was near the Wall of Darkness.
Regardless, Nydru's body convulsed in agony, and froze in a contortion of pain. His skin and clothing began to disintegrate, revealing himself to be an empty shell. Absolution is supposed to rip a soul out of its body, rendering the target temporarily or permanently dead. What happened with Nyrdru was that there was essentially nothing to rip out, he burned through in several areas with orange light and within moments there was nothing left but a smoldering heap of cloth. The point was that The Unlife had utterly destroyed his soul. He was "consumed by his insatiable master," the ultimate fate of anyone who would turn to it for power. Bandur Etrevion is portrayed the same way under certain conditions.
Behind The Scenes
Nyrdru would have been born after the Wars of Dominion, if he was human, but there were surviving servants of the Master of Malice in Gaalt. The Master was a former servant of Kadaena who was slain by Andraax, the battle which drove Andraax insane. In fact, Andraax vanished for thousands of years, traveling through the Portal at Ruuth. This was a powerful gateway to the other side of the Wall of Darkness. It is possible Nyrdru has an implicit back story with these servants, having come to be enthralled by The Unlife through them. It was a few millennia before the Raven Queen, a Dyari sorceress intrigued with (the I.C.E. form of) Fash'lo'nae, established her monarchy in the northeastern continents.