Ragnarok

Ragnarök is the Norse name for the end of the world.

In Mythology
Ragnarök is a series of future events, including a great battle foretold to ultimately result in the death of a number of major figures (including the gods Odin, Thor, Týr, Frey, Heimdallr, and Loki), the occurrence of various natural disasters, and the subsequent submersion of the world in water. Afterward, the world will resurface anew and fertile, the surviving and returning gods will meet, and the world will be repopulated by two human survivors.

The Sword of Summer
Odin is assembling an army, the Einherjar, to help him for Ragnarök.