Which god avenges Odin's death at Ragnarök?

Víðarr is the god who avenges Odin’s death at Ragnarök.

During Ragnarök, the wolf Fenrir swallows Odin in battle. Odin’s son Víðarr then attacks the wolf and kills it, avenging his father. Accounts describe him using a specially reinforced shoe or foot to hold Fenrir’s lower jaw while forcing apart the wolf’s jaws and striking it through the heart or throat.

Víðarr is known as a powerful, silent god. His role is relatively minor in surviving myths before Ragnarök, but his decisive act makes him one of the gods who survive the catastrophe. After the old world is destroyed, Víðarr and his brother Váli are among the gods who live on in the renewed world.

Vali is an easy mix-up because he is also Odin’s son and is associated with vengeance—especially Baldr’s death. Vali, however, is not the god who kills Fenrir and avenges Odin; that deed belongs to Víðarr.