You could call Chaos the beginning but he is also the end. You could call it some sort of a Cosmic Primordial Soup but he can also be an entity. I call it a proto-primordial because it perfectly describes who it is. So I am gonna go ahead and say that Chaos is a woman for convenience sake and because in Greek mythology, a woman is the only person that can perform parthenogenesis (Asexually reproduce just by thinking) . So, she is the embodiment of the void, the mother to all and a proto-primordial. We could also be the cause of the big bang as modern scientist calls them.
At first there was only Chaos. Then, from her, two beings were born. Nyx and her fated husband and brother Erebrus. Erebrus stands for “Deep Darkness” and “Shadow”, which completely suited his personality and primordial domains. This tale was written first in Hesiod’s Theogony.
Nyx is the Primordial of night. According to some myths she drives the “Night Chariot” across the sky to signify night after every day. But day came later. Hemera or Day was born as the daughter of Nyx and Erebrus, which is ironic because both of the parents are related to darkness while Hemera is… bright…
So, who or what is Chaos? Chaos according to me is a deity, an entity, which is in charge of the universe and the Primordial Soup also called Chaos. She is like Hades, both a god and the name for the underworld realm. Chaos could also just be a conscience who felt the need of introducing new species and making them thrive.
But I told you that she is also the end. This is because everything started with Chaos and everything will end with her. That is her impending doom, even though Moros (the personification of impending doom) would also be doomed during this circumstance (pun very much intended). Chaos is the maker, molder and the destroyer. Chaos, even though this sounds acronymous, started with peace and tranquility and will end with peace and tranquility.
She has created the universe like a potter making their pot and she will end it by breaking the already existing pot. We are just inscriptions engraved on the pot that are removed and new ones made every day. For how long? Only Chaos herself knows.