The fire giant destined to kill Freyr at Ragnarök is Surtr. In Norse mythology, Surtr is a powerful jötunn associated with fire and the fiery realm of Muspelheim or Múspell.
The Poetic Edda’s Völuspá foretells Surtr’s arrival from the south carrying a bright sword. It then says he will battle “Beli’s bane,” a poetic name for Freyr because Freyr previously killed the giant Beli. The poem does not give a long account of their duel, but it clearly places the two opponents together at Ragnarök.
Snorri Sturluson’s Prose Edda supplies the famous explanation for Freyr’s defeat. Freyr had given his magical sword to his servant Skírnir as part of the arrangement that won Gerðr as his bride. Without that weapon, Freyr must fight Surtr with inferior arms and falls after their struggle.
Surtr’s role extends beyond killing Freyr: after the gods’ battles, he sends fire across the world. Bergelmir, Hrungnir, and Thrym are different giants from other myths and are not Freyr’s Ragnarök opponent.