https://www.dndbeyond.com/monsters/ygorl-lord-of-entropy
A mote of alien thought given form and flesh, Thruun is not of this world. He dwells in an icy prison, awaiting a time when the world will be right for his coming. As his age of freezing darkness draws near, his brood begins to appear, stalking the wastes in preparation for his reign. If he is released from his prison, the entire world will be covered in a deadly glacier, removed from light and hope for all time.
Imprisoned somewhere in the Spine of the World with a caller held beneath the stone in Dougan's Hole.
The ancient elves spent much of their time in near-endless research and study. When the world’s mysteries were unraveled, they turned their minds to what lies beyond. The vast and varied planes of existence fascinated them, yet they still sought farther. The most powerful among them wanted to know what lay beyond the planes. As part of their work, the elves constructed a great many gates and portals to other worlds and planes of existence. In attempting to peer beyond the veils of infinity, they finally built their greatest creation: the Gate of Worlds, designed to pierce the boundaries of the planes. What they found destroyed them.
Legend holds that the Gate of Worlds opened a rift in reality, allowing a glimpse into a place known only as the Far Realm. This place of alien geometry, maddening thoughts, and toxic natural law was antithetical to mortal life. Even as the elves tried to close the gate, strange things slipped through, forever tying the world to the Far Realm. Although many of these entities have surfaced over the years only to be banished or destroyed, some remain to spread their foul taint. The records are far from clear, but the name of Thruun appears more than once.
Creatures of the Far Realm do not obey natural laws; their alien nature is deadly to those who come in contact with them. However, the world can affect them in strange ways as well. Ancient records made by the Gate of Worlds survivors note that some of the Far Realm entities could not live in the world. Some died unnatural deaths, while others merely ceased to function, going into a sort of dormancy until conditions would be right for them to thrive. One such entity seemed to have an aversion to light.
Although dormant, Thruun still proved deadly. Those fooled by his illusions were soon tainted by his corruption and had to be destroyed. Although exposing him to light caused dormancy, it also caused his form to radiate waves of deadly cold. Soon, sheets of ice began to engulf his body. The elves moved him to a mountaintop, where his chill would not harm anyone and the light of the sun might keep him dormant forever. The few remaining caretaker elves eventually died, and Thruun was forgotten.
Legends about Thruun grow vague beyond the confines of his mountain prison. Some rumors hold that the last great age of ice can be attributed to him, when darkness covered the land and he was nearly freed from his lethargy. One tale speaks of icy alien creatures that wear the tattered clothing of mountain barbarians and prowl the high passes, bringing freezing darkness with them wherever they go.