You earn Eternity fragments from arena rewards or buy them from the arena store.
Eternity souls guarantee a 5* character among the group we call "the eternals".
All three allies have a fury skill instead of a passive skill.
It's important to note that you get a 100 Frontier coins for each Eternity soul opened.
Eternity and Wrath are needed to collect Magma Beast Eddie from the Infernal Frontier dungeon.