Mimir

The wisest god of the Aesir. During the peace between two warring tribes of gods, the Aesir and Vanir, the two sides exchanged hostages. The Aesir received Njörd (Njord) and Freyr, while the Vanir received Mimir and Hoenir. When they discovered that Hoenir only seemed wise due to Mimir secretly giving Hoenir advice, the angry Vanir had Mimir decapitated. Mimir's head was returned to the Aesir. The head was preserved and Odin often used it to gain wisdom. See the War Against Vanir and the Head of Mimir.

There was a different story on how Odin gained knowledge. Beneath the Yggdrasill (World Tree) was a well called Mímisbrunnr (Well of Mimir). In order to drink the water from the well and gain knowledge, Odin had to sacrifice one of his eyes. See Sacrifice: Hanging and Runes.