Who was giant slain by Odin?

The giant slain by Odin is known as Ymir. In Norse mythology, Ymir is considered the primordial being from whose body the world was created. Odin and his brothers, Vili and Vé, killed Ymir and used his flesh to form the earth, his blood to create the seas, and his bones to make the mountains. This act set the stage for the creation of humanity and the cosmos in Norse belief.