Well, that didn't take long! Yesterday, we learned Game of Thrones star Kit Harington was signing on to the Marvel Cinematic Universe in a mystery role, and as of today's Marvel presentation at D23, the mystery is over! Harrington has signed on to co-star in Eternals. And the actor is taking on a very familiar role for comics readers: Dane Whitman, aka, Black Knight. While the character is not an Eternal himself, Whitman has a wild and fantastical backstory befitting a film about the otherworldly heroes. Per his comics history, after inheriting a family castle, Dane discovers …