At the destruction of the universe, Jormungand and Thor will kill each other.
In Norse mythology, Jormungand is one of the three children of the god Loki and his wife, the giantess Angrboda. The gods were well aware that this monster was growing fast and that it would one day bring much evil upon gods and men. So Odin deemed it advisable to render it harmless. He threw the serpent in the ocean that surrounds the earth, but the monster had grown to such an enormous size that it easily spans the entire world, hence the name Midgard Serpent. It lies deep in the ocean where it bites itself in its tail, and all mankind is caught within his coils.