Marvel's Black Knight doomed the Kingdom of Camelot, and was personally responsible for the end of King Arthur's reign. The legend of King Arthur has been adapted into countless mediums, including a classic Disney animated movie and the recently-released Netflix series Cursed. No surprise, it's also been incorporated into the Marvel Universe.
Stan Lee and artist Joe Maneely created the first Black Knight back in 1955. Sir Percy of Scandia lived at the time of Camelot, and he was chosen by Merlin to protect Arthur from a terrible threat. Sir Percy was given the mystical Ebony Blade, a weapon that made him almost invincible in battle. The Black Knight series only lasted five issues, but Stan Lee liked the idea, and in the 1960s he brought Sir Percy's ancestor into the Marvel Universe. Eternals will introduce this present-day Black Knight, Dane Whitman, who will be played by Kit Harington.
In 2007, Marvel Comics retold the story of the first Black Knight in the Mystic Arcana miniseries, and they revealed he was actually responsible for dooming Camelot and bringing King Arthur's reign to an end. Sir Percy had always been special, and when he was just a boy a druidic prophet foretold he would be "the Hand of Fate." That prophecy led to his being trained as a knight, and he caught the eye of Merlin, who saw his potential. And so Merlin chose Sir Percy as his champion, unaware he was fulfilling that ominous prophecy. Once Sir Percy had proven himself, Merlin took him into the caverns beneath Camelot, and showed him the Starstone - a crashed meteor that Merlin had Sir Percy forge into mystical objects while under the influence of a trance. Sir Percy had to choose between the four objects he had made. Only one could save Camelot.
Thus was the prophecy fulfilled. Sir Percy had become the Hand of Fate, because the choice he made that day would decide the future of Camelot. "A shield protects -- a staff smites -- and a chalice can give cool solace," Sir Percy observed. "But I am a Knight, and so I choose - a sword!" But those who live by the sword will die by the sword, and with that decision Black Knight had set Camelot down a path that led to war. In the end, foul treachery led to King Arthur's death at the hands of Sir Percy's own cousin Mordred, and prepared to ambush the Black Knight with a dagger also forged from the Starstone, thus able to kill him.
But as Mordred prepared to strike, Sir Percy received a vision of two futures. He could choose to live, and Camelot would endure, but it would become a corrupt kingdom that oppressed the world. Or he could allow Mordred to kill him; Camelot would fall, but the world that followed would be filled with both good and evil, with heroes and villains, light and darkness. As the Hand of Fate, Sir Percy was the one who would decide which of these two futures came to pass. Black Knight chose wisely, allowing Mordred to murder him, and so brought an end to the age of Camelot - but established the Marvel timeline as we know it.