Asgard has to deal with many planet killing foes. Surtur is but the tip of a huge iceberg. Here are a few enemies that can destroy Asgard if they wanted to.
There was a time when an alien race wronged Odin. In retribution, he compressed thatr race into one being. The Mangog is fueled by the combined hatred of billions of sentient beings. It is a being literally crafted with the Odin-Force.
The Frost Giants are a powerful race that are constantly at war with Asgard. They wield some powerful elemental magic and even more powerful physical attributes. When they band together, they lay siege to entire realms. Ymor is the greatest Frost Giant of all time. he is so large and powerful he is considered a literal force of nature on the same level as Surtur.
Ragnarok is essentially Thor but more evil and fiercer. He was created as a cyborg clone of Thor. Tony Stark used a strand of Thor’s air and used the DNA to create it. The clone was fit with the latest in weaponry and technology by Reed Richards and Tony Stark, the two greatest minds in of Marvel. Ragnarok has an insatiable hunger for violence and would easily destroy Asgard if he wills it.
Kurse is a Dark Elf monster with unimaginable strength. Thor has defeated multiple versions of the monster over the years. But the one we are talking about is Alvin The Strong. This version of Kurse was so strong it defeated Thor, Loki, and Beta Ray Bill singlehandedly. After the Beyonder multiplied his strength by a factor of four, his world destroying powers have only increased further.
Cul aka The Serpent is Odin’s brother and the Asgardian God of Fear. Despite managing to fend him off a long time ago, Odin knew Cul would one day surely return. When the banished God did come back, Odin was so scared of him he contemplated razing Earth to teh ground to prevent his brother from gaining any foothold.
The Midgard Serpent
The Midgard Serpent is immortal, invincible, and is destined to kill Thor when Ragnarok happens. It is also the largest monster Thor has ever faced in combat. No monstrous creature, not even Surtur, comes close.
Perrikus And The Dark Gods
Perrikus and his gang of scheming Dark Gods managed to overthrow Odin and take control of Asgard. When Thor came to their rescue, Perrikus slashed through Mjolnir and forced Thor to revert back to his human form. Only with the aid of the destroyer armor and Hercules did Thor manage to beat him.
The ruler of the Dark Elves is always scheming to overthrow Asgard and conquer the nine realms. Malekith is a master of dark magic, manipulation and has a very strategic mind. Think of him as a deranged Dark Elf version of the Joker, but with the addition of dark faerie magic. He is also the one who chopped of Thor’s arm. So he means business.
In recent issues, Thor becomes the Herald of Galactus and realizes the truth about Galactus. Galactus, who is known for making Heralds, is himself a Herald to a larger entity. The Black Winter is pretty much like Galactus. The only difference is while Galactus devours planets, Black Winter harvests an entire universe.