The demonic knight known as Necromon was an enemy of Captain Britain and the citizens of Camelot and a villain in the series is published by Marvel comics, making him by default one of Marvel's many supervillains.
Necromon's exact origins are unrevealed, although he does appear to be some sort of demon and is an old enemy of the natives of the neighboring realm of Otherworld and he sought to overtake that realm, as well as the planet Earth.
Knowing that the forces of Camelot would rally against them, Necromon sought to remove some of their champions as threats. Necromon caused madness and amnesia to overtake Brian Bradock (the current Captain Britain), causing him to leap from a plane and hide out on a remote cave. He also sent one of his agents Mordred The Evil and his Goblins to slay the Black Knight and the amnesiac Braddock. The two were assisted by Vortigen, one of the Proud Walkers, who helped Braddock regain his memory, and explained the nature of the attacks to them. After numerous battles with many of Necromon's agents, the Black Knight and Captain Britain succeeded in entering Otherworld to join the forces of Camelot.
Necromon ultimately led his warriors in an assault on Camelot. While Merlyn, the Proud Walkers, and the warriors of Camelot battled his forces, Necromon scaled the wall of the castle. Arthur Pendragon and the Black Knight confronted and battled Necromon, drawing him into a sword fight. Excalibur proved stronger than Necromon's sword, and King Arthur shattered his blade. The two warriors then backed Necromon into a threshold of the castle and the Black Knight sent a spiked gate crashing down on him. Necromon stopped the falling gate,but while he was distracted the two warriors charged forth and impaled him with both Excalibur and the Ebony Blade
Dying, Necromon used the last of his strength to destroy the castle of Camelot, causing it to fall into the abyss on the edge of Otherworld.