Sinu the Red was a lich, who served as the primary antagonist during Season 10 - To Slay a Lich, despite being first introduced way back in Episode 24 - Maps and Mystery. Jaela was tasked by her god, the Raven Queen, to destroy the lich, which she took a while to do. Looked a lot like the Lich in the Dungeon Master's Guide, but without the head-wear. Long claws and tattered red robes.
Sometime around the year 1300, Sinu was a human wizard who was taken in by an older wizard, Nictis Cross. Cross began torturing the boy, making him do horrible things, such as kidnapping young children to perform experiments on. Eventually, he was able to escape, and swore to do good in the world, to make up for all the bad, the pain he caused. After a while, he was called back to Nictis Cross. For the first time, Sinu felt fear. He blacked out. When he awoke, he could not remember anything that happened during the blackout, but in his journals, he writes that he slowly and horrifically tortured Nictis Cross to death. Throughout the rest of Sinu's long life, anytime he experienced any form of anxiety or fear, he would turn into a monster, and perform terrible acts. Eventually, the dark side of his mind took over, and he became fully a monster.
He was able to find his sister, who had their mother's old locket. Sinu killed his sister, and ended up with the locket. He used her soul to create a phylactery, a sort of soul-holder, out of the locket, to become a lich, an immortal and powerful evil creature. It was rumored that Sinu teamed up with the Drow and Duergar of the Razortooth Mountains, to create a fortress, The Crypt of Sinu the Red. There, he kept The Orb of Endless Power (which was used to bring back Aludra Wyrmsbane, after she died for the first time).
The Ätlän-tã Fælcons went to the Crypt of Sinu the Red to destroy Sinu once and for all, during the year 1907. Before they get there, the good half of Sinu's mind creates the illusion of a small village, Tarry Hollow, that existed long ago, in Sinu's youth, but has been destroyed. The town represents several aspects of his personality. The phylactery that contains his soul becomes a young boy, Teddy Dozar, whom the rest of the townsfolk want to find. Each night, the town is attacked by a horde of undead monsters, that represent the evil, monsterous side of Sinu. If they were to find the locket, Sinu would be immortal forever. Eventually, after a lot of searching around, the Fælcons found Teddy, and the illusion was revealed. Sinu appeared before them, and there was a intense battle. Toby used Sinu's robe to jump into the Astral Plane, where he grabbed a statue of a giant head, and returned to the Material Plane to smash the statue on Sinu's head. His spirit entered the phylactery, and Jaela finally fulfilled her mission in destroying him, by shattering the locket with her Raven Dagger. And thus, Sinu's reign was ended.