In Norse mythology, gods and goddesses usually belong to one of two tribes: the Aesir and the Vanir. Throughout most of the Norse tales, deities from the two tribes get along fairly easily, and it’s hard to pin down firm distinctions between the two groups. But there was a time when that wasn’t the case.
The Vanir goddess Freya was always the foremost practitioner of the art of seidr, the most terribly powerful kind of magic. 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” (“Gold-greed”) 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 (pronounced roughly “HIGH-neer”) and Mimir went to the Vanir.
Njord and his children seem to have lived more or less in peace in Asgard. Unfortunately, the same can’t be said of Hoenir and Mimir in Vanaheim. The Vanir immediately saw that Hoenir was seemingly able to deliver incomparably wise advice on any problem, but they failed to notice that this was only when he had Mimir in his company. Hoenir was actually a rather slow-witted simpleton who was at a loss for words when Mimir wasn’t available to counsel him. After Hoenir responded to the Vanir’s entreaties with the unhelpful “Let others decide” one too many times, the Vanir thought they had been cheated in the hostage exchange. They beheaded Mimir and sent the severed head back to Asgard, where the distraught Odin chanted magic poems over the head and embalmed it in herbs. Thus preserved, Mimir’s head continued to give indispensable advice to Odin in times of need.
The two tribes were still weary of fighting a war that was so evenly-matched, however. 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.
Sources: norse-mythology.org, Sons of Vikings, Children of Ash and Elm