Einherjar

The Einherjar are an army of elite warriors that serve as the main army of Asgard, and the main warrior-class of Asgardian society. Einherjar warriors are personally trained by the ruler of Asgard and are tasked with protecting Asgard and quelling conflicts within the Nine Realms. During times without war the Einherjar mostly serve as palace guards, notably serving as guards for Odin's Vault.

History
The Einherjar were first seen organized under one of Asgard's first rulers, Bor, serving as protectors of the realm of Asgard under his rule. As the kingdom of Asgard grew in influence within the nine realms it took on the role of keeping the peace between realms, thus the Einherjar were further tasked with policing the Nine Realms, preventing any realm from threatening any other realm's natural way of life.

Eons ago, during a cosmic event known as the Convergence, the Dark Elves, led by Malekith the Accursed, attempted to destroy the universe using a weapon known as the Aether in a bid to return the universe to its original state of darkness. However, they were met with resistance by the armies of the Einherjar, lead by the then current All Father, Bor. The two sides engaged in a prolonged conflict with the Einherjar fighting the armies of the Dark Elves, including their empowered "Kursed" warriors. The Asgardian army eventually stopped Malekith, staling the Aether from him using the Bifrost. However, a majority of the remaining Einherjar died as Malekith sacrificed his fleet of war ships, dropping them from the sky onto the warriors on the battlefield below.

Over time, Asgard rebuilt the forces of the Einherjar, who served in many more conflicts in other realms over the millennia before leadership was passed from Bor to his son, Odin, the current All Father of Asgard.

In 965 A.D. an army of Frost Giants from the realm Jotunheim invaded Tønsberg, Norway, in an attempt to subjugate Midgard(Earth) and plunge it into a new ice age using the Casket of Ancient Winters. However, they were opposed by Odin and an amassed army of Einherjar warriors, who, after a pitched war, drove the Frost Giants back into the heart of Jotunheim and took the Casket, forcing the Frost Giants into a peace agreement.