Eternals' Post-Credits Sets Dane Whitman Up As MCU's Evil Thor Mirror
Eternals' end-credits scene sees Dane Whitman, aka Black Knight, find the Ebony Blade, establishing his future as a dark mirror to the MCU's Thor.
Dane Whitman discovers the Ebony Blade in Eternals' end-credits scene, setting up an arc that positions him as an evil mirror to the MCU's Thor. Dane (Kit Harington) has a relatively small role in Eternals, serving as a romantic interest for Sersi but not just as a character with any great powers of his own - yet. It's clear from the movie, though, that Dane has a big MCU future, which Eternals post-credits scene in particular plays a major role in.
The Eternals' after-credits scene with Dane should come to define his future in a few key ways. There is, of course, the voice of Blade (Mahershala Ali), questioning whether he's ready for the Ebony Blade and setting up a possible team-up between the pair. There's the mention of Dane's uncle, something referenced a couple of times during the movie, which further establishes his family lineage as being important. And there is the Ebony Blade itself, connected to both and yet the most crucial in its own right, which can cement Dane's future as Black Knight.
The Ebony Blade is an ancient, mythical sword, and it's with this that Dane's arc will serve as a dark mirror to Thor's. The Ebony Blade is essentially the anti-Mjolnir; where the latter seeks worthiness, the former rewards those who are most impure of heart. The Ebony Blade amplifies the worst characteristics of a person, feeding on the most evil parts of them and their most negative thoughts, which in turn allows both the blade and its user to grow stronger. It drives them further down this path, leading to a lust for violence and bloodshed. In Marvel Comics, this was the fate that befell Dane's uncle, Nathan Garrett, who held the mantle of Black Knight before him, and it's something Dane of the comics has succumbed to on occasion as well. For the MCU, then, it means Dane's journey will be about disproving his "worthiness" to wield the sword, flipping how Thor had to prove himself worthy of Mjolnir.
Much like how Mjolnir comes enchanted by Odin, who cast that "whosever wields this hammer, if he be worthy, shall possess the power of Thor," the Ebony Blade carries a curse, as Eternals' post-credits scene makes clear: "Death is my reward." Thor's journey in the MCU has shown how even the idea of worthiness that been something of a curse for him, a near-impossible standard to live up to and part of his greatest triumphs and failures. The same test will be true of Dane as well, as he takes on the mantle of Black Knight and shows whether he is pure, or if he will give in to the sword's demands.
Across the MCU, Mjolnir has been positioned as one of the most powerful weapons in existence, and in particular a conduit for Thor's powers as the God of Thunder. The same could be said for the Ebony Blade, which is incredibly strong in its own right - it is near-indestructible, can cut through almost any material, deflects magic attacks, and makes the wielder something close to invulnerable. Thor's arc ultimately saw him prove his worthiness and power even without the hammer; Dane's story after Eternals' credits will, presumably, see him do something similar but in reverse, proving he can use he Ebony Blade even in spite of its corrupting, corrosive influence, but not without a few dark, or even evil, turns along the way.