The vikings believed that whenever a warrior died a hero's death, they would be taken into the sky by a Valkyrie, an Asgardian warrior maiden who rode on a winged horse. The Valkyrie would take a dead warrior to Valhalla, which was like heaven for the vikings. Valhalla was where everyone who died a warriors death went to. On Valhalla you could fight all you want and if you got killed, you could come back to life.
The Asgardians have enemies, like the frost giants and dark elves. The frost giants had always despised Odin because he was a being far more powerful and great than they, he was more civilized than a savage frost giant. Asgard and Jotunheim, the realm of the frost giants, have been at war several times.
The dark elves are from the realm of Svartalfheim and are ruled by the evil trickster, Malekith the Accursed.