Surtr appears in only a handful of genuinely old sources, chiefly a few stanzas of the Poetic Edda poem Voluspa, which describes him coming from the south with fire at the world's end. The fuller, more narratively complete account most people know, Surtr's home in Muspelheim, his role leading the fire giants against the gods, and his final duel with Freyr, comes from Snorri Sturluson's Prose Edda, written in the thirteenth century, roughly two hundred years after Iceland's conversion to Christianity and by an author working from a Christian literary and educational background even as he preserved older pagan material.
Scholars of Old Norse religion generally treat Snorri as a genuinely valuable source, since he clearly drew on earlier poetry now otherwise lost, but also as a systematizer who imposed more narrative order and coherence on scattered, sometimes contradictory eddic references than pre-Christian oral tradition necessarily had on its own. Some researchers have also asked how much the overall shape of Ragnarok, a final cosmic battle followed by a cleansing destruction and a renewed world, may owe something to the Christian apocalyptic framework current in Snorri's own time, a question that remains open rather than settled, since the pre-Christian oral material behind his account cannot be recovered independently to check against it.