actually, that happens wel before. When the Gods want to tie Fenir down and finally find the material to do it (made from the blood and intrails of his own kin btw) Tyr places his fist in Fenir’s mouth as a gurantee that they will free him should Fenir he unable to break loose. Fenir’s stuck, Tyr looses his hand and Fenir gets locked up. At Ragenok he kills Odin.