Erebos (aka Erebus) was an entity of pure darkness. Classified as an Old One it has no form and may be older than creation itself.
Tales differ as to whether Erebos grew at the same time, and in opposition to, Tartarus. Or if Tartarus grew inside Erebos like a parasite. Not that it matters.
Erebos is as much a place as it is a thing, it is the manifest awareness, and totality, of the deep abyss. A total darkness from which lesser spirits form and are reabsorbed, and the source of the spirit world.