Tyranian the Betrayer
Variously known as Tyranian the Fallen, Tyranian the Thrice-Damned, and Tyranian Blackheart, this infamous figure in ancient Elven lore was not always so reviled. The eldest son of King Elisterian of Keltamyre, he was a celebrated High Elven Bladesinger of the Circle of the Phoenix whose exploits during the wars with the Eldrakyn were the stuff of song and legend. When Elisterian was assassinated and Keltamyre fell, Tyranian took charge of the survivors and led them on a grueling march to Vallithyre, where they were welcomed with open arms.
Almost as soon as the Keltamyran Elves arrived in their new home, the High Magisters of Vallithyre began recruiting new blood to replace the Magisters who had perished in the ongoing skirmishes with the Dark Elves. Tyranian took up the call, and left his Bladesingers in the hands of his younger brother, Lysandir. It quickly became clear that he outstripped his peers, and he became the youngest High Magister in the history of the kingdom. As a High Magister, Tyranian was tasked with the maintaining of the magical palings that protected Vallithyre from the dark magic of the Illythiri. He created an ingenious system of runestones along the kingdom's leylines to bolster its defenses, and it had a remakable effect.
Tyranian was lauded as a hero, and basked in the adoration of his people, but at some point his desire to prevent Vallithyre from falling like Keltamyre turned to base lust for power. He craved an even deeper understanding of magic, and soon even his unparalleled command of the High Magic of his people wasn't enough. It is unknown precisely when he turned away from the Orlythae and began to worship the hated Spider Queen, but she seduced him in short order with promises of untold power and immortality. And when the High Magisters assassinated the Valsharess of Miyeritar, Tyranian's fury at what he deemed a cowardly act of murder knew no bounds. He turned his back on his people once and for all, allying himself in secret to Zhandura the Merciless, and conspired with the Ara'yathrin to bring down his own kingdom. Tyranian systematically dismantled the layers and layers of magical protections upon Vallithyre, hiding his work within new, useless palings that by all appearances were sound, but were in fact illusory. When Zhandura struck with her Ilhar Velve, Vallithyre was left utterly defenseless, and was annhiliated.
No one knows precisely what happened to Tyranian after his betrayal of the Elven people and the subsequent destruction of Vallithyre. He was widely assumed to have perished in the genocide, to the extent that the gathering of the Elven Court that met immediately following the the Descent of the Drow only condemned him in absentia in a symbolic fashion. But rumors persisted for years afterward that he did in fact survive, and was in hiding. Some even believed he followed the Dark Elves into exile in the Underground, but this was never substantiated.
Regardless of his ultimate fate, Tyranian is the most hated High Elf who ever lived, a source of tremendous shame for the High Elves even in the modern age. To this day, his name lives on in the Mithrandic term tyraniathér, considered unanimously to be the worst perjorative in the Elven tongue. It roughly means, "to trade one's soul for power", though it loses a great deal in translation.