At one point in the midst of his Court's war with the Seelies, he and the Seelie Queen had a secret truce and sealed it with a union, and she soon conceived a child. They agreed to have their offspring unite and inherit the Seelie and Unseelie thrones. They also wanted the child to be powerful enough to destroy the Nephilim, and they made it so. They used rites and spells, including dark magic, cast by magicians from all over Faerie, to give the child gifts, to make the perfect heir—"perfectly beautiful, a perfect leader, inspiring of perfect loyalty" "and perfect love, who would be braver than any faerie knight that had gone before." When the child was born a girl, who would be named Auraline, he was displeased. Nonetheless, the King kept her with him in his Court.
When a prophecy about the First Heir came about, one in which all of Faerie would fall under shadow when they reached their full power, the King decided to have her killed. Her daughter soon fled their world, seeking refuge in the world of mortals. Fearing the magic the child had and her own offspring would inherit, as well as the claim they would have over his throne, the King had her and her family hunted down. The King spread word that his daughter had been kidnapped and murdered. The elder faeries of Unseelie have since mistrusted him, noticing that he did not try hard enough to find her. Bitter over losing his chance at a perfect heir to a daughter, he had since had any female offspring killed, though the widespread belief was that it was only because he had no use for them.
The King eventually came to have around fifty sons with many different women, including Kieran with a water faerie. While many of his sons vied for the throne, he came to notice that Kieran was popular with the Court, particularly spurred on by another son and supposed favorite Erec. Due to these Court politics, he surrendered young Kieran to the Wild Hunt.
When the Seelie Queen gave birth to a child, named Ash, with the demon-blooded Shadowhunter Sebastian Morgenstern, the King had the boy kidnapped, wanting to continue with the boy what he'd started with the First Heir. He sought the Black Volume of the Dead to perform more spells on Ash and made plans to get ahold of it through manipulating Malcolm Fade or the Queen herself. He also opened a Portal to Thule to let that world's essence poisonous to Nephilim seep into theirs.
The rising conflict against Shadowhunters and the Seelie Court led to a coup d'état to be staged by his sons Kieran and Adaon. He was killed by Kieran with an elf-bolt, and though that made Kieran his rightful heir, he was at first succeeded by Oban.
The Unseelie King had a tall and lean build. He had two large, white horns curling from either side of his temples. He also had a deep pallor, with such white skin, and black hair. He wore a gold band on his head.
The contrast of the sides of his face made it seem like it was divided down the middle. The right side was the face of a handsome young man—clean and uncannily beautiful—while the left was heavily disfigured, with tight, gray skin and several scars, as well as an empty eye socket.
- The Land I Lost (mentioned only)
- Lady Midnight (mentioned only)
- Lord of Shadows (first appearance)
- Queen of Air and Darkness
- Forever Fallen (mentioned only)
- Secrets of Blackthorn Hall (alluded to)