What creature does Thor kill at Ragnarök before dying himself?

The story behind the answer

Thor kills Jörmungandr, the Midgard Serpent, at Ragnarök. He crushes the world-encircling snake with Mjölnir but is fatally poisoned, staggering nine steps before he dies. By contrast, Fenrir devours Odin (and is slain by Víðarr), Surtr leads the fire giants and burns the world, and Níðhöggr gnaws at Yggdrasil’s roots and survives.

This duel is foreshadowed in the fishing story where Thor nearly hooks Jörmungandr from Hymir’s boat. Memory tip: Thor (thunder) vs. the world’s serpent—when thunder breaks the world’s “loop,” it still gets bitten.