-- Warning: Major SPOILERS for Thor: Ragnarok lie ahead --
Thor: Ragnarok is an absolute blast from start to finish, but while its final act showdown makes for an immensely satisfying spectacle, it's possible that the title character's plan to take down Hela once and for all has merely given birth to an even greater evil. Over the course of the film, the Goddess of Death proves to be the most formidable foe Thor has gone toe-to-toe with to date. After being revealed as the firstborn child of Odin, she effortlessly destroys Thor's hammer Mjolnir and singlehandedly takes over Asgard by wiping out its standing army. Even once The God of Thunder and his fellow Revengers make it back home to confront Hela, she proves herself to be even stronger than they'd realized. (See: Thor telling Loki that he'd hit her with "the biggest lightning bolt in the history of lightning bolts and it did nothing.") Even with a Hulk on their side, Odinson's big sister looks all but unbeatable. So what's a Thunder God to do?
Summon an even more powerful/evil foe to take her out, apparently. When it becomes clear that the Revengers simply don't have the firepower to defeat Hela, Thor sends Loki to Odin's Vault to place Surtur's crown into the Eternal Flame. The fire demon -- whom Thor had nonchalantly bested in the film's opening scene -- is then reborn as a giant, one who now has the power to (seemingly) kill Hela and enact Ragnarok by simply plunging his sword into the ground. Again, the heroes voluntarily powered up an immortal supervillain to the point where he’s capable of destroying an entire realm in one fell swoop. That’s an infinity stone-level feat right there.
Related: Thor Ragnarok’s Ending Explained
So Thor's homeworld goes boom, Hela is defeated, and the remaining citizens of Asgard float away safely into space, having swapped an evil queen for a genocide machine... Hurray?
In the pages of Marvel Comics, Surtur is almost exclusively associated with Ragnarok, Asgard's eternal cycle of apocalyptic destruction and rebirth. The possibility of the realm's reincarnation isn't made all that clear in the film, but then, neither is Surtur's fate. It's tough to spot him floating in space amongst the ruins of Asgard -- admittedly, we were a bit distracted by Korg's perfectly timed rebuilding joke -- but unless he disappeared into nothingness after fulfilling his prophesied destiny, then Thor and the gang may have just unleashed an unfathomably powerful evil upon the galaxy.
Comic Surtur is a being that's even older than other immortals like Odin and Thor, and he's far stronger. His sword Twilight is the key to his power in the course material (rather than his crown, like in Ragnarok), and he can basically destroy anything he wants with it. So unless Surter was either wiped out along with Asgard or he returned to his home realm of Muspelheim to brag about his super awesome achievement, the remaining realms could have a serious problem on their hands.
All that said, the supervillain's future film prospects seem unlikely. The idea of following up a fun-filled romp through space with another apocalyptic battle with a personality-less fire demon feels like a longshot. If the Thor franchise does move forward, Surtur's existence probably won't even be acknowledged. The MCU has no problem with sidelining characters and toying with comic continuity. Plus, Thor and his fellow Asgardians look to have their hands full with a certain other all-powerful supervillain at the moment. Sorry, Clancy Brown fans, but you've likely seen the last of this fire demon in the MCU.