Introduction
Mimir is a figure of great wisdom, keeper of a well beneath the roots of the world tree Yggdrasil whose waters grant knowledge to those who drink from it. Beheaded in the aftermath of the Aesir-Vanir War, his severed head was preserved by Odin, who continues to consult it for counsel.
The well of wisdom
Mimir's well lies beneath one of the three roots of Yggdrasil, and its waters are a source of wisdom and knowledge so valuable that Odin sacrificed one of his own eyes to be allowed a single drink from it.
Beheading and preservation
As part of the hostage exchange that ended the Aesir-Vanir War, Mimir was sent to live among the Vanir alongside Hoenir. When the Vanir grew dissatisfied with Hoenir (finding him unable to give useful counsel without Mimir's guidance), they beheaded Mimir and sent his head back to the Aesir. Odin preserved the head with herbs and spoke charms over it, allowing it to continue speaking and offering him wisdom and secret knowledge from beyond death.
Kinship with Odin
The Prose Edda implies that Mimir is the brother of Bestla, Odin's mother, both children of the giant Bölthorn, which would make Mimir Odin's maternal uncle, a family connection that helps explain the exceptional trust and value Odin places in his counsel even after death.