Odin and his two brothers, Vili and Ve, kill Ymir in the account both Eddas preserve, and what follows is not disposal but construction: the three gods take Ymir's body and build the entire physical world directly from its parts. His flesh becomes the earth itself; his blood, welling out in a flood large enough that Snorri's Prose Edda says it drowned all but one pair of the other frost giants, becomes the sea and all its lakes and rivers; his bones become the mountains; his teeth and broken bone fragments become stones and boulders; his skull, raised up and held in place at its four corners by four dwarves, becomes the dome of the sky; and his brains, cast upward, become the clouds. The Poetic Edda poem Voluspa gives a compressed version of the same act, the seeress reciting the world's construction from Ymir's flesh as part of her wider prophecy of the cosmos's whole history from creation to Ragnarok.
The dismemberment-into-world pattern Ymir's death establishes is not unique to Norse myth in world mythology broadly, comparable cosmogonies built from a slain primordial being appear in a number of other traditions, but within Norse cosmology specifically it is the single founding act that makes everything else possible: without Ymir's death there is no earth for Midgard to stand on, no sky for the sun and moon to cross, no sea for later monsters like Jormungandr to inhabit. Ymir's single body is, in the most literal sense the myth offers, the material the entire visible world is made of.