Well consider the Abyss sort of like Paradise in the beginning (maybe Heaven is more relate able), it was the beginning and end of every thing, a world bathed in light and the creator of souls, governed by an entity known as the Nucleus of the Abyss, which didn't do much more than exist within the dark heart of the Abyss. Then the Intention was created when Levi impregnated Lacie in order for her to give birth in the Abyss and offer the Nucleus a companion/vessel, thus destroying this harmony of the Abyss and creating more of a chaos of the Abyss that we know today. Much more on the origins of the Abyss are unknown.
As for the Baskervilles, they are composed completely of individuals whom had been chosen by the Abyss in order to act as its 'messengers'. Chosen by droplets of light which are absorbed by the body and making the individual more Chain-like than human. The reason for Baskervilles existences is so that they can protect the harmony of the Abyss and the Chains which prevent Earth from falling into the Abyss as a whole. Long story short, the Abyss/ Nucleus one day decided it needed a guardian, and so it sent out droplets of light which were extensions of its power to choose those worthy of the task (i.e. those born with some form of distortion around their souls). What your asking about the Abyss is yet to be explained, and will likely not be explained further than it as been (Abyss being the beginning/end of all things as we know it).