Spoilers for Thor: Ragnarok ahead. Of all the Thor movies, Thor: Ragnarok is the one that changes our hero the most, both mentally and physically. Not only does he get a haircut (thanks, Grandmaster), he's also sans his famous red cape for a huge chunk of the film and loses his hammer, Mjolnir. But Thor's hammer isn't the only thing he loses in the film. Thor loses his right eye in Ragnarok during the final showdown with Hela, but it's not the first time he's been down this particular body part. In fact, Thor lost an eye in the comics in a storyline that connects to the film in an unexpected way.
Unlike in the comics' Ragnarok arc, in which Thor dies, the God of Thunder ends Ragnarok alive to fight another day. However, Thor's physical condition at the end of the film might reference a different comic book arc: Rune King Thor. In the film, Thor loses his eye going up against Hela, but eventually survives the war and Ragnarok (aka the destruction of Asgard). But, just when it looks like she might kill him once and for all, the pain and desperation he feels gives him a rush of power. Losing an eye sparks Thor's ability to channel his power of thunder without Mjolnir, allowing him to become a human conductor, hence the sparks coming off his body as seen in a few of the trailers. It's Thor like we've never seen him before on screen.
Thor's impressive new power reminded some fans of Rune King Thor, a version of Thor from the 1998 series The Mighty Thor: Disassembled. In the comics, Thor sacrificed to gain more power. And one of those sacrifices, as noted by The Insider, was to rip out his own eye to learn more about Asgard. (Coincidentally, his father, Odin, lost his eye in a similar way, exchanging it for knowledge of the future in the comics.) After participating in a few other challenges, Thor becomes Rune King Thor, a sort of all-powerful version of the hero, who is pictured with glowing eyes.
Thor is also depicted without one of his eyes in a more recent King Thor run of the comics, as seen in Thor: God of Thunder. Written by Jason Aaron with art from Esad Ribic, the series features the character of King Thor (aka Old King Thor). As the last living god of Asgard, Thor is now old and grey, and a bit worse for wear. Not only is he down an eye, he's also down an arm, which fans might recall was a fate almost determined for him in Thor: The Dark World when Loki cut off Thor's hand.
Though Thor's history in the comics might set a precedent for him losing an eye, it doesn't dictate what happens next for the character. It's unlikely we'll see Thor follow in the exact footsteps of Rune King Thor or King Thor. However, they do give us clues as to what Thor's eye removal means going forward. First, Thor losing an eye signifies his shift from God of Thunder to King of Asgard. After three movies, he has finally settled on the throne of Asgard and accepted the responsibility of being King, like his one-eyed comic counterparts and his father. The loss of his eye coupled with his new thunderous power might also lay the groundwork for a stronger Thor in Avengers: Infinity War. Thor's new powers will undoubtedly come in handy in the fight against Thanos.
There is one final concern Thor's lost eye raises: his mortality. As a god, Thor is not immortal, but he is on his way to living for thousands of years. However, the permanent loss of an eye is a sign of his mortality, and proves him to be much more fragile than fans might have thought. It's no secret that some of our favorite Marvel Cinematic Universe heroes will likely be meeting their ends in the final two Avengers films, and if Thor's lost eye in Thor: Ragnarok means anything, it's that Thor could very well be on the chopping block.