Aesir: The primary "pantheon" or group of Norse gods (led by Odin) - live in Asgard
Vanir: The secondary "pantheon" or group of Norse gods (led by Frey)
The two groups warred against each other early in time, but after that lived peacefully.
In preparation for the time when the riders of Muspelheim, the Land of Fire, with the Giants and the evil powers of the Underworld, would bring on battle, Odin All-Father was gathering a host of defenders for Asgard. They were not of the Æsir nor of the Vanir; they were of the race of mortal men, heroes chosen from amongst the slain on fields of battle in Midgard, the World of Men.
To choose the heroes, and to give victory to those whom he willed to have victory, Odin had battle-maidens that went to the fields of war. Beautiful were those battle-maidens and fearless; wise were they also, for to them Odin showed the Runes of Wisdom. Valkyries, Choosers of the Slain, they were named.
Those who were chosen on the fields of the slain were called the Einherjar. For them Odin made ready a great Hall -- Valhalla, the Hall of the Slain, it was called.
Five hundred and forty doors had Valhalla, and out of each door eight hundred Champions might pass. Every day the Champions put on their armor and took their weapons down from the walls, and went forth and battled with each other. All who were wounded were made whole again, and in peace and goodly fellowship they sat down to the feast that Odin prepared for them. Odin himself sat with his Champions, drinking wine but eating no meat.
For meat the Champions ate the flesh of the boar Sæhrimnir; every day the boar was killed and cooked, and every morning it was whole again. For drink they had the mead that was made from the milk of the goat Heidrun, the goat that browsed on the leaves of the tree Læradir. And the Valkyries, the wise and fearless battle-maidens, went amongst them, filling up the drinking-horns with the heady mead.
Youngest of all the battle-maidens was Brynhild. Nevertheless, to her Odin All-Father had shown more of the Runes of Wisdom than he had shown to any of her sisters. And when the time came for Brynhild to journey down into Midgard he gave her a swan-feather dress such as he had given before to the three Valkyrie sisters--Alvit, Olrun, and Hladgrun.
In the dazzling plumage of a swan the young battle-maiden flew down from Asgard. Not yet had she to go to the battlefields. Waters drew her, and as she waited on the will of the All-Father she sought out a lake that had golden sands for its shore, and as a maiden bathed in it.
Now there dwelt near this lake a young hero whose name was Agnar. And one day as Agnar lay by the lake he saw a swan with dazzling plumage fly down to it. And while she was in the reeds the swan-feather dress slipped off her, and Agnar beheld the swan change to a maiden.
So bright was her hair, so strong and swift were all her movements, that he knew her for one of Odin's battle-maidens; for one of those who give victory and choose the slain. Very daring was Agnar, and he set his mind upon capturing this battle-maiden even though he should bring on himself the wrath of Odin by doing it.
He hid the swan-feather dress that she had left in the reeds. When she came out of the water she could not fly away. Agnar gave back to her the swan-feather dress, but Brynhild had to promise that she would be his battle-maiden.
And as they talked together the young Valkyrie saw in him a hero that might prove himself worthy of Valhalla. Very brave and very noble was Agnar. Brynhild went with him as his battle-maiden, and she told him much from the Runes of Wisdom that she knew, and she showed him that the All-Father's last hope was in the bravery of the heroes of the earth; with the Chosen from the Slain for his Champions Odin would make battle in defence of Asgard.
Always Brynhild was with Agnar's warriors; above the battles she hovered, her bright hair and flashing battledress outshining the spears and swords and shields of the warriors.
But the gray-beard King Helmgunnar made war on the young Agnar. Odin favored the gray-beard King, and to him he promised the victory. Brynhild knew the will of the All-Father. But to Agnar, not to Helmgunnar, she gave the victory.
Doomed was Brynhild on the instant she went against Odin's will. Never again might she come into Asgard. A mortal woman she was now, and the Norns began to spin the thread of her mortal destiny.
Sorrowful was Odin All-Father that the wisest of his battle-maidens might never again appear in Asgard, the realm of the Gods. He rode down on Sleipner to where Brynhild was. And when he came before her it was his, and not her head that was bowed down in sorrow.
For Brynhild knew now that the World of Men was paying a bitter price for the strength that Asgard was collecting for the last battle. The bravest and noblest were being taken from Midgard, the World of Men, to fill up the ranks of Odin's Champions. And Brynhild's heart was full of anger against the Gods of Asgard, and she cared no more to be one of them.
Odin looked on his unflinching battle-maiden, and he said, "Is there aught thou wouldst have me bestow on thee in thy mortal life, Brynhild?"
"Nothing but this," Brynhild answered, "that in my mortal life no one but a man without fear, the bravest hero in the world, may claim me for his wife."
All-Father bowed his head in thought. "It shall be as you have asked," he said. "Only he who is without fear shall come near thee."
Then on the top of the mountain that is called Hindfell Odin had a Hall built that faced the south. Ten Dwarfs built it of black stone. And when the Hall was built he put around it a wall of fire.
But Odin All-Father was not yet done: he took a thorn of the Tree of Sleep and he put it into the flesh of the battle-maiden. Then, with her helmet on her head and the armor of the Valkyrie upon her, he lifted Brynhild in his arms and carried her through the wall of fire. He laid her upon the couch that was within the Hall. There she would lie in slumber until the hero without fear should ride through the flame and waken her to the life of a mortal woman.
He bid farewell to her and he rode back to Asgard on Sleipner. He could not foresee what fate would be hers as a mortal woman. But the fire Odin had left went blazing and circling around the Hall that the Dwarfs had built. For ages that fire would be the fence around which Brynhild, once a Valkyrie, lay in sleep.