The primordial void of Ginnungagap was the first state of existence, a chasm of infinite potential where nothing yet existed, no earth, no sky, no gods, no giants. When the heat from Muspelheim met the ice from Niflheim in this void, the first being, Ymir, was created from the melting ice.
Ymir was a hermaphroditic being who reproduced asexually, creating the first generation of giants from his own body. From his armpits, a male and female giant emerged, and from his legs, a six-headed son was born. He was nourished by the milk of Auðhumla, the primeval cow, who sustained him with four rivers of milk.
However, Ymir's reign as the first being was destined to end violently. The three sons of Borr: Odin, Vili, and Vé, slew Ymir and used his body to create the world. His flesh became the earth, his blood the oceans and lakes, his bones the mountains, his teeth the rocks and stones, his hair the trees and vegetation, and his skull the dome of the sky. The maggots that crawled in his flesh became the dwarves.
Ymir's death was the necessary sacrifice that made creation possible. His body became the physical world, and his descendants, the jötnar, would forever be in conflict with the gods who had killed their ancestor. This act of creation through destruction establishes the fundamental tension in Norse mythology between order (the gods) and chaos (the giants), between the created world and the primordial forces that preceded it.