Vortigern was an early villain of Arthurian legend, ruling before Uther Pendragon.
He didn’t become king by birth; while Constans, the eldest brother of Uther, was king, Vortigern secured a marriage into royalty. As King Constans was still quite young, Vortigern installed himself as the king’s advisor, and conspired to have the young king killed.
Constan’s younger brothers, Aurelius Ambrosius and Uther Pendragon, were children at the time, so once Constans was dead Vortigern seized the crown for himself and ruled with a tyrannical fist.
Day 20: King Vortigern