The Vanir goddess Freya was always the foremost practitioner of the art of seidr, a form of magic principally concerned with discerning and altering the course of destiny. Like historical seidr practitioners, she wandered from town to town plying her craft for hire.
Under the name Heiðr (“Bright”), she eventually came to Asgard, the home of the Aesir. The Aesir were quite taken by her powers and zealously sought her services. But soon they realized that their values of honor, kin loyalty, and obedience to the law were being pushed aside by the selfish desires they sought to fulfill with the witch’s magic. Blaming Freya for their own shortcomings, the Aesir called her “Gullveig” and attempted to murder her. Three times they tried to burn her, and three times she was reborn from the ashes.
Because of this, the Aesir and Vanir came to hate and fear one another, and these hostilities erupted into war. The Aesir fought by the rules of plain combat, with weapons and brute force, while the Vanir used the subtler means of magic. The war went on for some time, with both sides gaining the upper hand by turns.
Eventually the two tribes of divinities became weary of fighting and decided to call a truce. As was customary among the ancient Norse and other Germanic peoples, the two sides agreed to pay tribute to each other by sending hostages to live among the other tribe. Freya, Freyr, and Njord of the Vanir went to the Aesir, and Hoenir and Mimir went to the Vanir. Rather than renewing their hostilities over this tragic misunderstanding, each of the Aesir and Vanir came together and spat into a cauldron. From their saliva they created Kvasir, the wisest of all beings, as a way of pledging sustained harmony.