Chaos is the unknowable cosmic energy that birthed at the dawn of the Multiverse where only the Old Ones originally existed. From Chaos sprang the Primordial Deities, who in turn created the titans and thus the creations within that Multiverse. Chaos, as well as Entropy, became imprisoned within the Primordial Deity known as Tartarus where it would become fuel for the hecatoncheir, who were also spawned by Chaos.
Legends of the NeSiverse
Existing within Chaos, that substance or entity from which everything would come to exist, were the Old Ones that were little more than concepts. But from Chaos came the Primordial Deities, which the Old Ones observed as the creatures became the very fabric of the Multiverse itself, their essences interwoven into physics and metaphysics. They were seven; Nyx, Hemera, Erebus, Uranus, Eros, Gaia and Tartarus. Tartarus came to imprison Chaos within itself and too, eventually, Entropy[Leg 1].