Erebus

"Like a mighty fire casts brilliant light, so too does his very presence cast a darkness. A darkness so complete it swallows the very light of hope that once burned in men's souls."
~Shadows and Sorrows~

Erebus is said to be the personification of darkness and the Void itself, spawned by Chaos, the first of anything. Tartarus (the underworld), Gaia (Earth), Erebus ( darkness and the void between worlds,) Eros(love, life, and passion,) and Nyx (darkness and the night) together make the five primordial deities born of Chaos. Of these 5 deities came everything else, including other deities.

It is important to note that different gods often have similar aspects. Just as Ares and Athena are both gods of war, Erebus and Nyx are both gods of darkness. Erebus is the god of absolute and everlasting darkness, a death like aspect, while Nyx is the goddess of the temporary darkness which helps restore our world in the night an aspect more like that of rebirth. However unlike many gods with similar aspects Erebus and Nyx were allies, lovers, and had many offspring including Hypnos (sleep,) Thanatos (death,) the Moirai (the weavers of fate,)and literally thousands of others including many daemons.

Nyx

The gods of Olympus, generations younger than the primordial gods, so feared Erebus and Nyx that though their children often interfered in the plans of Zeus, even directly attacking him at times, retribution was rare and never in full force.

Erebus is often depicted as a demonic entity, with monstrous features and darkness radiating out from within himself. This makes some sense as both he and Nyx are considered to be "dark gods." This is more in reference to their children's personifications (including insolence, death, plague, and more.) However I think this highly unfair as their bonding also gave the world Aether (light) and Hemera (day.)