The Einherjar Lieutenant served in the Asgardian Army, the Einherjar, under King Bor's command during the battle against Malekith's Dark Elves in Svartalfheim. The Einherjar Lieutenant and his comrades followed their king's lead and were able to defeat Malekith's army.
Following the battle, the Lieutenant approached King Bor, and asked him what they should do with the Aether, the superweapon Malekith had intended to use to revert the universe to a state of primordial darkness. Knowing the Aether was too powerful to be destroyed, Bor ordered him to bury it on a distant world where nobody would be able to find it.
The Lieutenant complied to the order, hiding the Aether so well that it remained hidden for 5,000 years.