Hercules (1998)

Season 1: Episode 56

We see how the Greek Gods got their position when helping to build Greece. Morpheus' younger brother Phantasos makes a blanket of nightmares to drag through the night sky giving everyone nightmare because he should've been made The God of Sleep instead of Morpheus. The mortals' nightmares start to concern all the Greek Gods including Morpheus. It's up to Hercules and Phil to stop Phantasos' plot in their shared dreams.