Before he was killed during the Siege of Asgard, Loki manipulated Hela into taking his name out of the Book of Hel, meaning that he would never truly die, and he was now capable of returning as a ...

Excluding Freyr, the god introduced to the Norse pantheon through DLC, Loki is the only one who can access Hel. This makes Loki incredibly powerful in the late game, with access to strong myth units.

The following day Thor and Loki are about to return to Asgard in shame ... winds itself right around Jotunheim…and the old lady - 'Hel' - is actually death itself, which no-one can defeat.