Surtur is a lord of the Fire Demons from the extra-dimensional planes of Muspelheim.
Surtur desires the destruction of all the Nine Realms through fire and depends on his minions and subordinates to do what he pleases.
Surtur is a massive Fire Demon. His body is well-built and appears to have tremendous strength. His body is red and his hair is made of flames.
While Heimdall and Thor talked about the chaos that Loki has made to the Nine Realms, Heimdall feels that the seal of Bor's Sons was broken, he tells Thor that Odin must be informed about what just happened. After the Seal of Bor's Sons was broken, the fire demon lord escaped and burned Nidavellir to the stone and stole the shattered remains of the sword he seeked. When the last Norne Atone was destroyed by Wonder Man, Amora was sent to Muspelheim. Here, she founds Surtur, who imforms her that now she works for him.
Surtur then possessed Enchantress and attacked with his army the homeworld of Beta Ray Bill, Korbin. At the burning planet, he reforged the Twilight Sword. After the Enchantress was defeated by Thor and Bill, he cleared the forging of his sword. He later sent the Enchantress to free Loki and allow him to go on a rampage through the Destroyer as a distraction for the Avengers while "planting the seeds of destruction".
After Enchantress reports to Surtur, he then said that Ragnarok will descend and all will burn.
Thor was sent to Muspelheim when Nighthawk activated the contingency plan against Thor, during which a Norn Stone was attached to the God of Thunder's hammer, causing him to teleport through different realms, one of them being Muspelheim, where he saw Surtur. After being freed from the contingency plan by the Hulk, Thor mentioned that he flew through Muspelheim a total of 81 times. When the Bifrost was affected by the use of the Space Stone, Thor, Hulk and Hawkeye were accidentally transported to Surtur's realm. They were extracted out of Muspelheim by Heimdall's portal shortly before falling on Surtur's grasp.