If your knowledge of Norse mythology comes from the Marvel cinematic universe, in particular the Thor movies, you have a decent but imperfect education on the subject. There are things that the writers of those screenplays, and the comicbooks upon which those screenplays were based, changed for the benefit of the story. For example, the goddess Hela is not Loki’s sister in the original mythology; she is his daughter. Loki’s other children are Jormungandr the Midgard Serpent and the giant wolf Fenris, or Fenrir. You saw Fenrir get clobbered by the Incredible Hulk in THOR: RAGNAROK.
In the original tales, only the god Tyr dared to approach Fenrir to feed him, so big and nasty was the beast. When the gods sought to bind Fenrir with a magical chain by tricking him into thinking it was a test of strength for him, the wolf refused, unless one of the gods, as a show of good faith, would put his hand inside Fenrir’s mouth. Tyr agreed and slipped the chain around Fenrir’s neck, sticking his hand into Fenrir’s mouth, but when Fenrir realized that he couldn’t escape, he bit off Tyr’s hand. Tyr knew that this would happen, choosing to make the sacrifice to get Fenrir under control.