In Norse mythology, the einherjar are the souls heroic dead in Valhalla. The Vikings and Norse peoples believed that when warrirors died honorably in battle, they would be taken up to Valhalla by valkyries. At Valhalla, in the Æsir’s home of Asgard, they await Ragnarok where they will fight alongside Odin and the other Æsir.
Essentially, the einherjar are a select group of brave individuals chosen by Odin to assist him in the coming battle.
The name einherjar is Old Norse for “lone fighters”.