Is Surtr Ragnarok? In God of War, Yes, Alone

In God of War Ragnarök, Surtr does become Ragnarök, the huge fire-and-ice giant who smashes into Asgard. The prophecy said he and Sinmara would combine to form it, but the finished game gets around that: Surtr transforms alone, using Kratos's Blades of Chaos and Sinmara's heart. In Norse mythology he never becomes Ragnarök, since there he is only a fire giant who fights in it. (God of War Ragnarök)

How does Surtr become Ragnarök?#

Surtr is a fire giant who rules Muspelheim. By the time Kratos and Atreus reach his forge, he's spent ages avoiding his part in the prophecy. He and Sinmara had learned that Ragnarök would require them to merge into a giant and die together. So he separated from her, and they traded hearts so each kept part of the other. (God of War Ragnarök)

He refuses to help Kratos at first, because the prophecy would cost Sinmara her life. What changes his mind is the Blades of Chaos. He senses primordial fire in them, imbues them with his own fire, and tells the pair that joining the blades to Sinmara's frozen heart will make him Ragnarök. He insists Sinmara stay out of it. (God of War Ragnarök)

He leads them to the Spark of the World, where the primordial realms join, pulls more primordial fire from it into the blades, and tells Kratos to combine the blades with Sinmara's heart. Kratos stabs them into the heart, Surtr's transformation begins, and he falls from the path. (God of War Ragnarök)

Do Surtr and Sinmara merge to become Ragnarök?#

In the final game, no. The prophecy says they would, and the God of War Wiki states the prophecy that way, but the workaround Surtr finds lets him transform alone. The wiki adds a detail worth knowing: Sinmara survives, and Kratos, Mimir and Freya can hear her grieving in Niflheim afterwards. (God of War Wiki, Sinmara; God of War Ragnarök)

So if you've read "Surtr and Sinmara merge to become Ragnarök", that describes the prophecy, not what the game ends up showing. It also describes material that was cut. The wiki says fans found it in the game files, and Screen Rant credits YouTuber Omega Fantasy with turning up the unused subtitles. (God of War Wiki, Surtr; God of War Wiki, Sinmara; Screen Rant, cut Sinmara ending)

What happens to Ragnarök after the transformation?#

Ragnarök speaks from out of sight and tells Atreus he awaits the call, which Kratos and Atreus can make with the Gjallarhorn. In the final battle, Ragnarök advances on Asgard even as dwarven war machines try to stop him, and Freya and Freyr slow him down while the Midgardians are evacuated. After Odin's death he thrusts his sword into Asgard's heart. The explosion destroys Asgard and kills him as well. (God of War Ragnarök)

How is it different in Norse mythology?#

In the myths, Surtr (the name means "black" in Old Norse, per the wiki) is a fire jötunn who is foretold to lead the fire giants against the gods at Ragnarök. He carries a bright flaming sword, is destined to kill the god Freyr, and the flames he unleashes then engulf the Earth. Until then he guards the entrance to Muspellheim. (God of War Wiki, Surtr)

That's a giant fighting in a war. Nothing in that description has him turn into the war. The game's version, where he is the apocalypse, is its own invention.

Canon Notes#

My read#

The cut ending comes closer to the original prophecy: Sinmara merges with what's left of him, and both of their hearts are found after Asgard falls. The shipped version turns it into a loophole, and I think it's the better one, because Surtr's whole arc is about refusing to let Sinmara be a casualty. Whether it's what the writers first wanted I can't say, since the wiki only says the cut material was "likely" dropped to shorten the game. For the full account of the game's ending, see God of War Ragnarök's ending explained.