In Norse mythology, Mímir is the wisest of the Norse gods. Mímir possesses a well called "Mímisbrunnr" located at the second root of the Yggdrasil tree. And according to the legends, those who drink from this well will obtain more wisdom.
He is one of the ancient giant gods. He was known as the leader of the gods of that time, who according to some books was responsible for some unjust and life-taking deaths of people and others.
Mímir is mentioned in the poems of the Poetic Edda, Völuspá and Sigrdrífumál. In Völuspá , Mímir is mentioned in two stanzas.
Stanza 28 refers to the sacrifice Odin had to make (losing one of his eyes) to drink at Mímir's well, and states that Mímir drinks mead every morning "from the bet of the Father of the Dead (Odin)."
Stanza 46 describes that, in reference to Ragnarök, Mímir's "sons" are at stake as "fate burns" (though no additional information about these "sons" has survived), that the god Heimdall blows the Gjallarhorn, and that Mímir's severed head gives Odin advice.
The only mention in stanza 14 of the Sigrdrífumál is also a reference to Mímir's speaking with his head detached from his body. Stanzas 20 and 24 of the poem Fjölsvinnsmál refer to Yggdrasil as Mímameiðr.
In chapter 15 of the book Edda in Prose, in Gylfaginning, as the owner of the well, Mímir himself drinks and gains great knowledge. To drink from the well, he uses the Gjallarhorn, a drinking horn that shares its name with the horn used by Heimdall with the intention of announcing the beginning of Ragnarök. The section further relates that the well is located beneath one of the three roots of Yggdrasil, in the realm of the Jotun.
Chapter 51 reports that with the beginning of Ragnarök, "Heimdall rises and blows the Gjallarhorn with all his might. He awakens all the gods, and then holds an assembly. Odin now walks to the well of Mimir, seeking counsel for himself and his followers. The ashes of Yggdrasil quiver, and nothing, whether in heaven or earth, is without fear."
Also in the Edda in Prose, in Skáldskaparmál, Mimir's name appears in several kennings. These kennings include "friend of Mím" (for "Odin") in three places, "mischief-Mímir" (a kenning for "jötunn") and among a list of names for jötunn.
The character of Mimir appears as an ally in the game God of War 4, in which he is dubbed by Alastair Duncan.