A fire demon who exists to end Asgard, and who is entirely right about being able to.
Surtur is a fire demon, the lord of Muspelheim, and the figure in the prophecy of Ragnarok who is meant to bring Asgard down. He imprisons Thor in his lair, tells him that Odin is not on Asgard, and explains that if his crown is united with the Eternal Flame he will grow large enough to finish the realm. Thor beats him and escapes with the crown, which is the mistake.
He is motion captured by Taika Waititi, who directed the film, and voiced by Clancy Brown, and the opening scene is one long joke at the expense of villain monologues that turns out to have been the plot summary.
Minor spoiler
Thor works out that Asgard is a people and not a place, and sends Loki to put the crown into the Eternal Flame on purpose. Surtur does exactly what he said he would do, and the survivors watch from a ship.