Surtur is like the Asgardian version of the devil. Even Odin fears this guy, so he stuck him inside the Earth's core. Unfortunately he's been freed a couple of times. If there was one person you'd guess to let Surtur free, who would it be? If you guess Loki then you guessed correctly. Loki is always causing trouble.
Surtur was in the movie 'Thor: Ragnarok' in a rather faithful depiction. His legend was always that he would destroy Asgard. That ends up being true when Thor unleashes Surtur so that Surtur can destroy both Asgard and Hela, the goddess of death. It's like the classic saying goes, "if you can't beat them, call an ancient demon to destroy your world." It's Thor's version of pulling the cord on the video game console just to avoid losing.