Erebus was the god of darkness, well, that’s putting it simply. Remember, Primordial deities weren’t just the gods of xyz, they were the personification of xyz. Erebus WAS darkness. Theoi says it best. “(His) dark mists enveloped the edges of the world, and filled the deep hollows of the earth. His wife Nyx drew these mists across the heavens to bring night to the world, while his daughter Hemera scattered them bringing day : one blocking out the light of Aither (shining, blue heaven) and the other revealing it. The bright upper air (aither) was regarded as the source of day in the ancient cosmogonies rather than the sun.”
Pretty words. Creepy concept.
His wife, Nyx, is the goddess of night, and together the two had lots of kids, particularly of the Chthonic persuasion. These children included Aether and Hemera, mentioned above, the Hesperides, Hypnos, the Moirai, Geras, Styx, Charon, and Thanatos.
Erebus was also a realm of the Underworld where the living passed immediately after they died.
Erebus runs into the same problem a lot of the Primordial deities do in that he is not terribly active in mythology. It’s hard for the darkness to actually DO something other than, you know, be dark, but he’s pretty cool nonetheless.