Hypnos, the personification of sleep, was a primordial god who could allow mortals to forget all of their worldly worries by putting them to sleep while enabling them to have pleasant dreams. His twin brother Thanatos, the very incarnation of peaceful death, would come to collect and make sure the souls of the deceased were safely transported to the Underworld. The twins lived in a secluded cave palace where the River Lethe was said to flow across.