Spoilers for Black Knight: Curse of the Ebony Blade #2 ahead!
The Eternals will introduce the Black Knight into the MCU - and the comics have just revealed he is an ancestor of King Arthur. Dane Whitman, the Black Knight, has never exactly been an A-list superhero, yet he's one of the characters set to be introduced in The Eternals, played by none other than Kit Harington. Marvel's been gradually upping the Black Knight's profile in the comics ahead of his big-screen debut, making Whitman a key player in the battle against Knull and launching a miniseries, Black Knight: Curse of the Ebony Blade, written by Si Spurrier and featuring art by Sergio Davila.
Marvel's also been retconning the Black Knight, revealing the Ebony Blade is essentially the inversion of Mjolnir. Where Thor's enchanted hammer is drawn to those who are worthy, the Black Knight's mystic blade is drawn to those who are decidedly unworthy. As Dane told Thor, "It lightens only in shadow." In order to access the full power of the Ebony Blade, the Black Knight must choose to focus upon his own inner darkness; his rage, his grief, his pain, his wounded pride. When he does this, he is able to release the power of the Ebony Blade as what the Avengers describe as a WMD, one able to completely atomize mystical threats.
Black Knight: Curse of the Ebony Blade #2 now adds another twist to the tale, revealing Dane Whitman is actually an ancestor of King Arthur. It seems anyone descended from Arthur who wields the Ebony Blade gains a new ability; the power of resurrection. As the ghost of Sir Percy of Scandia explains it, "Something the wizard [Merlin] whispered over the Starstone as I warmed the forge [for the Ebony Blade], 'Who shall die yet holds this treasure... though the toll he cannot measure... if his blood be Arthur's strain... shall not rot -- but rise again."
Whitman is focused mostly on the new power of resurrection he has just learned he possesses, but he failed to note that Merlin is implying Sir Percy of Scandia, and all his descendants, are somehow related to King Arthur himself. Until now, it had seemed Merlin chose Sir Percy simply because he sensed he possessed the necessary character traits to wield the Blade, but it now seems he was also of a family line already rich in magic. This adds a whole new dynamic to the sword-and-sorcery tales of the Black Knight.
Black Knight: The Curse of the Ebony Blade is rewriting Dane Whitman's backstory, and it's clear the retcons are simply going to continue - and, indeed, will likely increase in scale. The most intriguing element of this particular retcon, though, is that it has been conducted in such a smart and subtle way - so much so that even the Black Knight isn't aware of it.