Surtr

"The sword will be mine, boy. You cannot change fate."

- Surtr to Magnus Chase during their fight on Longfellow Bridge, in The Sword of Summer

Surtr is the Jötunn-god of the Sun, Fire, Volcanic Eruptions, and the Cosmic Flames. He is the overlord of the fiery realm of Múspellsheimr, from which he had created the sun and stars of the Norse cosmology. He is the eldest of the Elder Jötnar - the eldest and most powerful jötnar who represent the primordial forces of nature, with him representing the primitive sun. He is also the king of the Fire-jötnar - a branch of jötnar who specialized in fire-related sorcery.

In the prophecy of Ragnarök, he is said to be the one who shall slay the Vanir god Freyr using his own sword Sumarbrander, and then combined it with his own sword Lævateinn to create the ultimate fiery sword of destruction that would burn down the ash tree Yggdrasill, destroying the entire Norse cosmology in the process. He is also doomed to be killed by the resurrected Baldr, who shall use his flames to recreate the world and restart the cycle of Ragnarök once more.

He is a part of the Götterdämmerung Alliance - an alliance formed by the Norse gods' most fearsome enemies who strive to bring forth the Twilight of the Gods and the apocalypse of the Norse Pantheon.

Biography
In Norse mythology, Surtr (Old Norse: "black" or "the swarthy one") rules the world of fire, Múspellsheimr, guarding the entrance of this realm with his shining sword, Lævateinn, that is said to be brighter than the sun. He is also said to be the creator and the destroyer, as the fires of his realm created the stars of the heavens, and later during Ragnarök, it will rain down fire unto all land.

The Ship of the Dead
Surtr was mentioned by Hrymr and Loki. They revealed that due to favourable tax laws, Naglfar was registered in Múspellsheimr and therefore Surtr is the technical owner of the ship. The two said he was forced to break up disagreement between them but the Elder Jötunn was not in Niflheimr.

I Play with Fire
While recruiting minor gods for post-Ragnarök reconstruction, he was being watched by Alex Fierro who was trying to sabotage his recruitment. When it looked like the gods had doubts about him, he summoned his fire sword to strike fear in them. Just then he was attacked by the child of Loki and had his nose cut off again. Meanwhile backup for Alex arrived and Surt was hogtied.