In Norse mythology, which monstrous serpent is a sibling of Hel?

In Norse mythology, Jörmungandr is a sibling of Hel. The serpent is one of the three famous children of Loki and the giantess Angrboða, alongside Hel and the wolf Fenrir.

The gods feared Loki’s children and dealt with them separately. Odin cast Jörmungandr into the sea surrounding Midgard, where the serpent grew large enough to encircle the world and grasp its own tail. This gave it the name Midgard Serpent or World Serpent.

Jörmungandr’s most important mythological opponent is Thor. Their conflict culminates at Ragnarök, when Thor kills the serpent but dies soon afterward from its venom. Jörmungandr is therefore linked to both the family of Hel and the final destruction of the old cosmic order.