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Warning! This page contains spoilers for The Sword of Summer.
Later, when Magnus, Samirah al-Abbas, Hearthstone, and Blitzen go to re-bind the wolf Fenris, Surt appears with an army of fire-giants and fights Gunilla and the other two Valkyrie, Irene and Margaret, killing them. After re-binding the wolf Fenris, Magnus sends Surt back to Muspellheim using Jack.
Surt was mentioned by Hrym and Loki. They reveal that due to favorable tax laws, Naglfar is registered in Muspellheim and therefore Surt is the technical owner of the ship. The two say he is forced to break up disagreement between them but the fire giant was not in Niflheim.
Surt is a sinister, cruel, remorseless and diabolical individual, to the point that he threatened to kill a group of innocent mortals, if Magnus had not gave sumarbrander to him. He is only interested in the start of the Ragnarok, so he can fulfil his destiny to burn all nine worlds to ashes.
Magnus describes Surt in The Sword of Summer as a human-sized dark man in a dark suit, with his skin being "the purest most beautiful shade of black [he'd] ever seen", blacker than squid ink at midnight. His clothes are the same color, and consist of a well-tailored jacket and slacks, a crisp dress shirt and tie—all seemingly cut from the fabric of a neutron star. In battle, Surt wears a trim-cut three-piece suit of chain mail, a tie and a shirt that appear to be woven from flame. Surt's chiseled face is "inhumanly handsome", and resembles obsidian, while Surt's long black hair is combed back in "an immaculate oil slick", and the pupils of his eyes glow red-hot, like tiny rings of lava. As a result, Magnus decides that Surt looks like a very stylish and well-dressed Satan. Surt's voice is described as deep and resonant, with his accent vaguely German or Scandinavian. He wields a fearsome curved sword made entire of white-hot flames. When wounded, Surt's sizzling blood glows like magma.
Being one of the first beings to came into existence, as well as the supreme ruler of Muspellheim, Surt is one of the most powerful and feared beings in the nine worlds, his power is so great, he is fated to burn all creation to ashes during the Ragnarok.
- Prowess in Battle: Surt is an extremely formidable master at combat, and is destined to command the forces of evil during Ragnarök. In The Sword of Summer, Surt held his own against three highly skilled Valkyries at once, (one of them Gunilla, who had 5 centuries of combat experience), eventually defeating and killing all of them in the process.
- Pyrokinesis: As the ruler of the Fire Giants of Muspellheim, Surt can generate vast amounts of heat and fire at will. He is destined to unleash his full power at the very end of Ragnarök, burning the Nine Worlds with his flames.
- Heat Generation: Surt can make the air around him "ripple with heat", to the point that he can walk straight through a car, melting a path through it "like a blowtorch through wax." He also made the paved ground bubble around him.
- Firestorm: When Blitzen and Hearthstone enrage him, Surt causes flames to swirl around him in a spiraling firestorm, melting cars to slag heaps, liquefying the asphalt, and popping rivets from the bridge like champagne corks.
- Fire Sword Summoning: Surt can summon his curved flaming sword at will. However, his blade dissipates when he temporarily loses concentration, like when Sumarbrander unexpectedly wounds him.
- Fire Immunity: Surt is completely immune to any amount of fire and heat.
- Enhanced Strength: As a Fire Giant, Surt has considerable superhuman strength, being able to back-hand both Hearthstone and Blitzen with ease, and punch Magnus hard enough to make the latter vomit and temporarily loose consciousness.
- During the events of Ragnarök, Surt is destined to kill Magnus' father Frey with Sumarbrander, and then use his powers to burn Asgard and the other realms.
- Surt is known to have caused the Great Chicago Fire.
- His Greek equivalent is Typhon.
- His Egyptian equivalent is Set based on their black/red hues and destructive storms (sand and fire).
- He is the inspiration for the Balrog from "The Lord of the Rings" series by J.R.R. Tolkien