Jormungandr

The Jormungandr is the son of Loki, the god of mischief, and a giantess according to Norweigian folklore. He was banished to earth's waters. The Jormungandr is a mighty sea serpent that grew so large it encompasses the entire world. It is also known as the Midgard (earth) Serpent.

When it releases its tail Ragnorak, the end of the world, will begin. Which I was looking forward to in Thor: Ragnorak. But holy fudgebutts, Marvel missed a good CGI opportunity. I was so disappointed. *sigh*

Anyway, the Jormungandr is the arch-enemy of the god Thor who I don't need to explain which powers he has thanks to the Marvel movies. In one story, Thor slays the Jormungandr.

In another when Ragnorak begins, Fenrir sibling to Jormungandr will set fire to one half of the world. You might know him better as the giant wolf Hela rode upon in Thor: Ragnorak.

The final encounter with Thor and Jormungandr is when Thor kills the serpent then walks nine paces and dies from its deadly venom.