They had to restrain a monster. Fenrir- the giant wolf. One of Loki's children. They tried two chains and he broke both of them, so they had the dwarves make a third one and dared Fenrir to let them chain him up. They promised they'd let him go right away, but he didn't trust them and he said he wouldn't do it unless one of them put their hand in his mouth.

[He rubs his wrist like it aches, which it does when he remembers this.]

Tyr stepped up when none of the other gods would, and when Fenrir realized the truth...