Hypnos

A sculptor recounts a two-year friendship built entirely around chasing deeper and stranger dream-states with a companion who seemed to already know the way there. Their explorations stop the night the companion goes further than the narrator is able to follow, and stop for good after a final, unwitnessed night that leaves the narrator's mind permanently damaged and his friend replaced, in the room where he'd been sleeping, by a marble statue carved with a single Greek word.