Ymir is the primordial Frost Giant who, according to Norse mythology, is one of the oldest beings in the cosmos, coming into existence when the Nine Worlds of the Norse cosmology were in their infancy. He begot the frost giants who were the ancestors of the Jotun, or race of the giants who later came to inhabit the realm of Jotunheim, and who frequently warred with the Asgardian deities. He was a being of fantastic stature and power that was equaled by a cruel and malevolent nature. Ymir factors heavily into the Norse myth of creation, which is, like all myths of creation (including the one in the Bible), a symbolic description of the celestial forces that created the universe we--humanity--calls home and within which our planet exists, along with all interconnected realms of reality (which includes the Nine Worlds of the Norse cosmology). Hence, in accordance with this fact, a description of the Norse creation myth is in order.
In the beginning, all that existed was an endless yawning void called Ginnungigap, though the energies in the nascent reality resulted in the creation of two oppositely polarized realms, Muspelheim, a realm of fire that existed to the north of Ginnungigap, and Niflheim, a realm of ice that existed to its south. Eventually, a few sparks of fire from Muspelheim traveled across Ginnungigap and melted some ice in Niflheim they came in contact with, thereby creating a liquid substance called eitr, which is said to have been composed of the building blocks of life. A multitude of eitr droplets coalesced together and formed into a gigantic humanoid being said to be composed of "rime frost," which was then hit by more sparks of energy from Muspelheim; these sparks of energy proved to be a catalyst for the life-giving bio-etheric properties within the eitr that formed the giant's body in much the same way as the lightning from the skies of the ancient Earth struck the antediluvian seas and catalyzed the life-giving organic molecules within it to form the first simple life forms; thus, was Ymir brought to life. During a period of sleep, he began perspiring under his arms, possibly shedding off excess amounts of eitr in his enormous form, and these droplets of perspiration combined to form two more giants of the opposite gender, and from the perspiration from his legs was spawned a third, male giant, Thrudgelmir. The three of these initial offspring of Ymir went on to reproduce sexually and spawn the race of Jutuns, the frost giants who were the ancestors of the Jotuns, the later race of giants. These early beings were nursed from the milk of the primal cow of massive size, Audumla, who was created from the melted ice of Niflheim and brought to life by sparks of catalytic energy from Muspelheim in a manner similar to that of Ymir.
As time passed, Audumla licked the salt-encrusted ice of Niflheim into the shape of a male humanoid being who also came to life, possibly as a result of being infused with eitr from Audumla's salivary glands and her own divine life essence. This being became known as Buri, the progenitor of the Aesir, one of the two tribes of deities that would eventually combine with another tribe, the Vanir, to form the Asgardians. Buri managed to spawn another deity, Borr, without a female mate, possibly by secreting perspiration filled with life-giving eitr in a manner similar to the way Ymir asexually spawned his own early offspring. In turn, Borr spawned three other male deities, the brothers Vili, Ve, and Odin. Wielding great combined power, these three members of the fledgling Aesir were appalled at the degree of evil and brutality promulgated by Ymir towards the vernal life forms of the multiverse, and they therefore engaged in a fearsome battle with the primal Frost Giant, finally managing to kill him. After doing so, according to the rest of the Norse creation myth, they bled the dying Ymir so that a massive flood of his life fluid spread across the land and wiped out almost the entirety of his frost giant progeny. The two exceptions were the son of Thrudgelmir--the frost giant Bergelmir--and his wife, who managed to hide in a huge hollow tree (presumably an early form of plant life that emerged out of the continuously created eitr). These two then subsequently birthed the race of giants that would later inhabit the otherdimensional realm of Jotunheim and become sworn enemies of the deities of Asgard, who were led by Odin and his wife Frigga. The three sibling deities then proceeded to carve up the amazingly massive corpse of Ymir to form the Earth (which they called Midgard), using the Frost Giant's teeth to form the rocks, his bones to form the mountains, and his brain matter to form the clouds in the sky; they then took embers from the sparks of Muspelheim to form the sun, the moon, and the stars. The maggots that emerged from the remainder of Ymir's corpse, obviously created asexually from the remaining eitr in his body somehow being catalyzed by the force of the Frost Giant's death, burrowed into the ground to quickly evolve into the diminutive humanoid race known as the dwarves, who dwelt in the subterranean regions of the realm known as Nidavellir. Finally, Vili, Ve, and Odin used Ymir's skull to form the heavens, and utilized four particularly powerful dwarves--Nordri (North), Sudri (South), Austri (West), and Vestri (East)--to hold up the heavens and represent the four cardinal points of direction and the four stags of the cosmic ash tree Yggdrasil, as well as representing the four winds in a manner similar to the four giants who represent the four directional winds in Greek mythology. Following the creation of the material manifested world of Midgard (Earth), the deities were able to use it as a dwelling place for the human race of mortals, whom they later created, to live free from the threat of the giants who were descended from Bergelmir and his mate.
Ymir has a page in this shrine to underscore his incredible importance to the Norse cosmological schema, as he represents the chaotic forces that eventually created the Earth in the universe that we know. He personifies the forces of the early Earth as it was four billion years ago, where sparks of energy (i.e., lightning) formed from the torrential storms of the Noachian skies to strike the live-giving elements then extant within the seas, thereby enabling life to form via a long forgotten form of spontaneous generation that evidently cannot naturally occur under the greatly changed conditions of the Earth as it is today. The remaining essence of Ymir still rumbles through the cosmos, a lingering aspect of the chaotic forces of an era long lost in the mists of antiquity, and these forces are dark and destructive enough that it's quite unwise to call upon them for magickal purpose in almost all circumstances. Nevertheless, the spirit of Ymir deserves its place in this shrine out of the matter of the primeval Frost Giant's sheer importance to the creation of the Norse cosmology, even if he isn't a being one would be quick to revere.