Mimir stands with the Aesir, the primary gods of Norse Mythology. He is the maternal uncle of Odin, King of the gods. Many of the ancient eddas and sagas  relate tales of this friend to Odin.  Like all the gods in Norse mythology Mimir is descended from the giants.  He is dark and swarthy like all that race.  Some consider him the father of Night herself.  The Hávamál (138-141) relates that once Odin wished for greater strength and wisdom. By hanging for nine days on the World Tree and offering his left eye,  he convinced wise Mimir, who guards the fountain of wisdom (Mimir’s Well) and  mighty spells,  to give him a drink from the precious spring and to teach him the spells.  

Which was a good thing for the Odin’s people, called the Aesir, because in the beginning of time two groups of gods battled for dominion over the nine worlds.  Odin led the Aesir forth from  to battle against their enemy the Vanir.  When they made peace, Mimir was one of the Aesir hostages sent to Vanaheim, the home world of the Vanir.  Chapters 4 & 7 of Heimskringla relate that the Vanir suspected the Aesir of cheating them in the exchange of hostages, so they cut off Mimir head and sent it to Asgard. Odin took the head of Mimir, embalmed it with herbs to make imperishable and spoke spells over it, which gave it the power to speak.  Odin kept Mimir's head with him and that it divulged the secrets of all the other worlds and that it gives wise counsel to Odin. 

But there’s something wrong with the stories of Mimir’s head.  

If Mimir is a god, by definition he cannot die. And if he is a giant, well even then the finality of death is not an axiom of Norse Mythology.  For even any mortal man who died in Midgard, that is Middle-Earth, the world of men would be received in one of three other worlds.  The Valkyries, winged goddesses of Death would pluck young lovers from those fallen on the battle field and fly them to Asgard and Odin’s great Hall called Valhalla where they would feast and fight to their heart’s delight.  Meanwhile with the van- goddess Freya’s other of their fallen brothers-in-arms would likewise eternally practice their martial arts in preparation for Ragnarok; the Twilight of the Gods.  When the forces of good and evil, order and chaos would meet for one final battle.  Those men who did not die in battle, but slain by old age or disease; were buried with boots and appropriate weaponry and sent off to a much gloomier underworld Hel to await the end of time.  

Likewise Mimir did not die. He could be found in a nicer corner of the underworld called Mimir’s Grove, where the woman and children found sanctuary.   The god of everything good, Balder, his wife and his brother also lived there.  Across the ways also waited the deceased giants.  Victor Ryberg in Teutonic Mythology calls Mimir’s realm a subterranean grove of immortality and asylum. In chapter 15 of the Prose Edda bookGylfaginning, as owner of his namesake well and grove, Mimir himself drinks from it and gains great knowledge. To drink from the well, he uses the Gjallarhorn, a drinking horn.  Ryberg list several activities of the supposedly dead Mimir; Mimir is mentioned as the guardian of Heimdal’s horn and other treasures like the sword forged by Thjasse, intended to destroy the world of the gods.  Mimir surprised the owner in his sleep and took the sword way to the lower world.  

At the beginning of Ragnarok, Heimdall stands up and blows the Gjallarhorn with all his strength. He wakens all the gods who then hold an assembly. Odin now rides to Mimir's Well, seeking council for both himself and his followers. Ryberg says, the firm friendship between Odin and this strange giant of the lower world was formed in time’s morning while Odin was still young and undeveloped and continued until the end of the gods and the world.  Whichwas at hand. 

The Giants attack Asgard.  The sons of Mimir take up their position at gates of the underworld: their long swords are in their hands. The wolf Fenrir eats Odin, Odin's son Vidar avenges his father by prying the wolf’s jaws apart and stabbing it in the heart with his spear. The earth-girdling serpent Jörmungandr fights Thor. Thor, also a son of Odin defeats it, but only takes nine steps before collapsing.  The hound Garmr; the  worst of monsters ights the war-god Týr, resulting in both of their deaths.    Loki fights Heimdall, and the two kill one another. At which the sons of Mimir cheer for they are no longer afraid. The Van-god Freyr fights the leader of the Fire-Giants;  Surtr and loses. Surtr covers the earth in fire.  Wolves drag the  sun and moon from the sky.  The stars disappear. The earth and mountains shake so violently that the trees come loose from the soil. The mountains topple.  The earths  sink into the sea. 

But, some of the gods and men did not die even then.

Earth will appear once more from the sea, beautiful and green. The world is decked in beauty. Fields yield produce without being sown. Where Asgard once stood, untouched by Surtr's flames,  stand Vidar and Vali.  Thor's sons Móði and Magni will meet them there, and, coming from Hel, Balder, his wife Nana and brother Hoder also arrive. Together, they all sit and recount memories, later finding the golden game pieces the Aesir once owned. The humans return from Mimir’s Grove and a new sun rises.   

And Mimir and his sons live on beside the Well of Wisdom. Mimir did not die.