The old norse manuscripts describe Surtr as the first of the fire giants to emerge from the flames of Muspelheim, the realm of fire. He's a primordial jötunn, half-brother of Ymir and Audhumbla, embodiment of chaos and fire. He's also represented as holding a immense fire SWORD
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During Ragnarök
In Ragnarök, Surtr will lead the army of fire jotnar from Muspelheim to invade Asgard by the Bifrost bridge and enter in the battle field.
Surtr is destined to fight with the god Freyr, who unfortunate is killed by the flame sword of the giant because he gifted his own sword to his friend Skírnir long before the battle.
In the end of the battle, Surtr will shake his weapon and spread the primordial Muspelheim fire to all the Yggdrasil, consuming the Nine Realms and killing practically all the living beings in the cosmos, including himself and his own species.
an opponent for Zoro