Floki (Gustaf Skarsgård) is Ragnar’s best friend and loyal companion. He’s a character that’s likely gone through the most ups and downs, as he experiences losing his family, the death of his best friend, and a momentary loss of faith in his gods.
In Season 5, Floki starts a small settlement in Iceland — a place he believes to be the land of the gods. Unfortunately for him, the same cycle of violence that always plagues his people starts in the settlement, and its residents start fighting and killing each other. Sick of the constant bloodshed, Floki travels into the cave of a volcano that’s erupting, believing this to be the gate to Helheim (aka Hell). Unfortunately, once he arrives, Floki sees a cross, and this breaks him. It would seem that the Christian God has already made it to the land of his gods. And as the volcano erupts around him, the last thing the audience sees is Floki laughing hysterically.
That is, until Season 6. Turns out, Floki is alive and well, living in Newfoundland and ready to greet Ubbe when he arrives. He kind of breezes over how he managed to escape his fate, simply noting that he survived the volcano incident and has been living peacefully with the native people.
According to Michael Hirst, he wanted to bring Floki back to give him a happy ending and “do right” by his character. He envisioned the series ending with characters looking out at the sea as it represented exploration to the Vikings, so that ending between Ubbe and Floki just felt right (via Express.co UK).