One of the more delightful things about the Thor mythology in the Marvel series is it ties closely both with old-school Norse mythology, but also a lot of the mythologies Lord of the Rings-type novels pulled their fantasy materials from. That's how the MCU winds up with Dark Elves, for instance. With parts of Avengers: Infinity War playing like a direct sequel to last year's Thor: Ragnarok, this brings some of those characters to the crossover film, like the planet Nidavellir, and characters like Eitri. Who is Eitri? And why did the MCU keep it a secret this was who Peter Dinklage would be playing? Warning: Spoilers for Avengers: Infinity War follow.
Eitri is the King of the Dwarves in Thor mythology. The reason the production kept it a secret is once fans knew Nidavellir would be one of the destinations in the film, they would know Thor would be getting back at least some version of his hammer, which was destroyed at the beginning of Thor: Ragnarok.
Nidavellir is probably not a name which is familiar to the casual fan, but hardcore Thor nerds would have recognized it instantly as one of the Nine Realms, the best forge in the galaxy and home to the hammer Mjølnir. The dwarves who worked as blacksmiths on Nidavellir were responsible for the creation of the legendary hammer.
Eitri is the ruler of Nidavellir, and the lead blacksmith of the hammer, after being personally asked by Odin to create it. In return, Odin promised to protect the dwarves and their realms, as well as Nidavellir. Though the dwarves forged weapons, they were not trained fighters. In the Thor: The Dark World game, Nidavellir is one of the locations attacked by Rock Trolls which needs protection.
But with Asgard having their own problems in the last few films, Nidavellir was left to fend for itself, and when Thanos attacked, they had no one come to their aide. As Eitri tells Thor, he made a terrible choice. In order to protect his people, he agreed to whatever Thanos wanted. In this case, the creation of the gauntlet strong enough to handle all six infinity stones. Once the gauntlet was created, Thanos went back on his word and killed the whole population. All except Eitri, who he declared worthy of living... just without hands, so he could never forge another weapon.
As it was, fans already were speculating Thor would forge himself a new hammer before the end of Avengers: Infinity War. (Though, to be fair, it could have been part of the beginning of Avengers 4.) Either way, both movies were occurring prior to any Ragnarok sequel (assuming there is going to be one), and Thor having his preferred weaponry was the only practical way for him to be able to fight Thanos.
As for keeping the character a secret, the production sure did a great job with it. When Dinklage was cast originally back in 2016, a few places speculated his role was part of the dwarf kingdom, and after Ragnarok took out the hammer, it made sense he might be on to forge a new one.
But just as many were convinced Dinklage was one of the CGI "children of Thanos" and his role was to be part of "Team Evil" against the Avengers on Earth. Marvel even teased images of the one CGI character who did not have an actor attached, and fans immediately took the bait in speculating this was indeed Dinklage, throwing everyone off the scent completely.
Either way, it was a well kept secret and Marvel should be applauded for being able to keep spoilers so tightly under wraps. We hope to see Eitri in Avengers 4.