Spoilers for Thor: Ragnarok ahead. Going into Thor: Ragnarok, the third and perhaps final Thor movie staring Chris Hemsworth as the hammer-wielding God of Thunder, a lot of fans had the same question: what in the world is Ragnarok? The filmmakers, it seems, anticipated this question, and in the very first scene of Thor: Ragnarok, a fire demon named Surtur is the one who gets the chance to explain it. Well, Surtur is a key player in the prophecy that predicts the total destruction of Asgard — a prophecy known as Ragnarok.
Thor: Ragnarok opens with Thor dangling in Surtur's cave home, captured seemingly on purpose in an attempt to learn more about Ragnarok and how to prevent it. In his rage, Surtur tells Thor that Ragnarok will happen when his powers are restored by lighting his crown (his horns) with the Eternal Flame, conveniently being held in the vaults of Asgard. It is Surtur's destiny, he believes, to destroy Asgard completely and reduce the planet to nothing but ash and dust. In the comics, the character of Surtur has always been tied to Ragnarok. In "Avengers: Dissembled," Surtur kills Odin while trying to bring about Ragnarok, according to io9. Later in the story line, Thor allows Surtur to destroy Asgard and fulfill Ragnarok, just as he does in the new film.
At the end of Thor: Ragnarok, Thor restores Surtur's strength using the Eternal Flame, seeing the fire demon as the only one capable of defeating Hela. Hela, Thor's surprise older sister, draws her power from Asgard itself, and unable to match her in strength, Thor decides to cut off her power source instead. Surtur's resurrection then unleashes the complete and utter destruction of Asgard, presumably killing Hela. In the end, the fire demon fulfilled Ragnarok, but also saved the day.
What's most unclear about Surtur is what happens next. In Thor: Ragnarok, the audience sees Surtur engulf Asgard in a ball of fire to the point where the entire world is obliterated. After the destruction, however, Surtur is nowhere to be seen. Despite multiple appearances in the comics, Surtur remains a mysterious figure in one very specific area: death. The fire demon, who ruled over the dimension of Muspelheim, doesn't really seem like the kind of creature that can be killed. Though Thor is able to rip his horn-crown off of his head at the beginning of the film, how he achieves this feat is never shown. Nor is it shown how Surtur is brought back to his full strength by the Eternal Flame. Keeping these events off screen raises the question: can Surtur really die?
How Surtur might survive such a massive explosion, we don't know. (Though, as an ancient being and fire demon, it seems safe to assume he has some tricks up his sleeves.) What we do know is that if Surtur is alive it could mean trouble for Thor down the line. The fire demon fulfilled his destiny by destroying the physical location of Asgard in Ragnarok, but he didn't exactly destroy Asgard completely. Before the end, Thor and company were able to successfully evacuate Asgard, which means that the Asgardian people are still alive. Ragnarok appeared tied to a physical place, but if Surtur is alive and if he ever finds out that Thor is attempting to build a new Asgardian home for his people, it's reasonable to think he might come back.
If Surtur does return, it's possible he could team up with Thanos. Both, it seems, have a certain affinity for causing death and destruction. And one can only imagine the challenge a Surtur-Thanos team-up would pose for the Avengers in coming films. But even if fans don't see the return of Surtur any time soon, it would be foolish to assume he is gone and defeated forever. Characters in the MCU tend to come back when you least expect them, and if Loki can cheat death, then so can a mysterious fire demon. Asgard might want to watch out.