Entropy is the great death of the Multiverse, the force to end all things both physical and metaphysical. It uses time to progress its affects on material reality. It is trapped within Tartarus, which limits its affects upon the Multiverse to a slow and gradual end.
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].