In Norse mythology Surt rules the land of fire, Muspelheim, guarding the entrance of this realm with his shining sword (Laevateinn) that is said to be brighter than the sun. It 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 Ragnarok, it will rain down fire unto all land.
Along with Astaroth and Arioch, Surt is one of the three major sub-bosses of the last arc of the game and one of the most powerful demons under Lucifer's orders that has taken control over the inferior part of the Basilica.
He appears defending the second floor basement, accusing the protagonist from being "nothing more than a mere puppet of God" before engaging him in battle if he's not chaos aligned. This fight, however, is entirely optional, as the player can choose to just take the stairs towards the next floor without having to enter Surt's room.
In the Chaos route, after locating the throne room of Asura in the last floor basement, the hero will need to go back to Surt's room, where he will give him the "Claw of Surt", a valuable item needed so Asura can forge the "Devil Ring" and the fight against Michael and his followers become available.
" A fire giant in Norse mythology. He rules the realm of fire, Muspelheim, and brandishes a sword of fire. In Ragnarok, he will set the world ablaze."
—Shin Megami Tensei III: Nocturne compendium info
Surt is one of the three Nihilo demons Hikawa enlisted to guard the entrance of the Diet Building while he seeks out the stored Magatsuhi there. The Demi-fiend will battle Surt immediately upon trying to enter the lower-left door.
Surt can later be encountered as a regular enemy in the Tower Of Kagutsuchi, but he cannot be recruited due to being a Tyrant Class demon. He can instead be summoned in the Cathedral of Shadows by fusing the right demons.
As a minion, Surt has the distinction of having the only fire-elemental based normal physical attack. His attack power can still be enhanced by Tarukaja and Focus, and when he is given the passive skill "Pierce", the effect will bypass resist, void and absorb properties of the Fire element instead of physical. When using a physical skill however, Surt will inflict standard physical damage and his normal attack can still be repelled by Tetrakarn. Surt also has a unique Fire spell aptly called Ragnarok.
"A giant of Norse mythology. His name means "black" or "jet black." Surt is the king and protector of the giant race of Muspel. He guards the entrance to Muspelheim, home of the Muspel, and is said to have been sitting on the edge of Muspelheim since before the world was created. Surt wields the magic sword Laevateinn, whose blade shines brighter than the sun. With it, Surt can reduce all around him to ashes in one stroke. At Ragnarok, the end of the world, Surt will defeat Frey, who wields a deer's horn. He is the only one of his race who will survive the battle, and will go on to incinerate the world in a sea of fire in preparation for its rebirth."
—Shin Megami Tensei IMAGINE Compendium
In the Chaos path in Envoy for the Messiah/Gaia Church, players are required to defeat him in the trial in Doukan's spirit world. Answering questions to his satisfaction will allow players to obtain the ability to fuse him and a smaller variant more appropriate for the level players are at at that stage. He is a special triple fusion of Jikokuten, Fenrir, and Shiki-Ouji. His sword, Laevateinn, can be acquired through a special synthesis recipe.
"A fire giant in Norse lore. He rules the fire realm, Muspelheim, and brandishes the burning sword Laevateinn. At Ragnarok, he will set the world ablaze."
—Strange Journey Compendium
Surt appears as a summonable and recruitable demon. He also appears as the first sub-boss of Sector Horologium in Law Path, destroying the elevator to Horologium B9F. Also, he appears in the EX MissionThe Wise Man's Library V, holding half of the last Book of Thoth.
Aside from returning as a Max Magician S. Link Persona, Surt can be fused with a Nihil Weapon to create the Laevateinn, a two-handed sword that inflicts fire damage instead of slash, and adds the Fire Boost effect as well.
Surt returns as the Ultimate Persona of the Magician Arcana. If the player takes the newly added female perspective, maxing out the Junpei Iori Social Link will allow the fusion of Surt. This is somewhat fitting, as Surt and both of Junpei's Persona (Hermes and Trismegistus) both have an affinity for fire.