Board Thread:Lore Discussion/@comment-24459775-20140315020817/@comment-16047389-20140512194129

The Abyss is the physical manifestation of Dark. The Darkwraiths constructed the Abyss when they began collecting Humanity, and bringing it to a single location. Manus did the same thing after he was reawakened. Since Manus' soul is infact a giant amalgamation of Humanity (leading to further evidence that it is infact the Dark Soul), it is possible that he constructed the Chasm of the Abyss from his own soul, but I believe it is more likely that he used the citizens of Oolacile. The Bloatheads, despite originally being people, do not drop Humanity. I believe that the corruption that they suffer from is the result of what happens when a human that is not afflicted by the Curse of Undead is drained of their Humanity.

The Abyss is not "nothingness". We know that creatures/beings both operate within the Abyss and, in some cases, are even born from it (and on top of that, we can clearly see a cobblestone floor underneath it if a strong light source, such as the skull lantern, is used in New Londo's Abyss). Since the Four Kings were the creators of the Abyss beneath New Londo, they and those that served them (the Darkwraiths) were unharmed by it. However, once the Chosen Undead slays the Four Kings, it is no longer under their control and is rendered inert. This is why after the encounter, even if the Chosen Undead is not a Darkwraith and not wearing the Covenant of Artorias, it will no longer attempt to claim them. It is likely that the Four Kings were attempting to spread the Abyss to the point where it would engulf all of Lordran (or atleast New Londo) in order to claim even more Humanity.

Since the Abyss is Dark in physical form, it is likely that those that have "become one with it" (those that have died) are able to manifest through it. How or why is a mystery, although it could be at the behest of whomever controls the Abyss at the time (i.e. Manus in Oolacile or the Darklurker in Drangleic). But we do know that humans and other beings return to the Dark when they die, and in some cases even crawl back out of it. This is heavily implied through dialouge with Grave Warden Agdayne. Additionally, I believe that the reason why Darkdiver Grandahl believes that the Abyss is needed "more now than ever" is because the Abyss is in fact an enormous collection of Humanity.