From everything that I have read/found/heard from npcs, the Great Swamp is a place in the world of the living, not at all present in game. Honestly, I think there is something to this whole sealing theory. Initially I assumed the sealer whose gear you take wasthere to heal the blighted populace simply because she had the power to. However, looking back on the item descriptions and the remedy sorcery, she was there to atone for some past sin... I believed this to be the sealing of New Londo, but you may be on to something here.WyrmHero wrote:I'm speculative to the theory because Blighttown has been always referred as the Great Swamp. It's just a pile of sh!t and disease that is home to the pyromancers. How the hell they lived there I don't have a clue.
The Demons weren't sealed in Blighttwon, they ran through Lordran but Gwyn's Black Knights slaughtered them in the chaos war. There seem to be a few survivors such as the Taurus Demon and the Asylum Demons but all the others were contained by the lava from Ceaseless and the Lordvessel yellow gate on the Demon Ruins entrance (near Demon Firesage). Quelaag, her sisters and brother escaped from Izalith when the Chaos Flame exploded and they settle near Blighttown and they were intelligent enough to not go any further.
While the are no visible mountains of corpses as we see in Drained New Londo , the buildings are clearly constructed in a way that suggests that the water level was once a great deal higher than it currently is in-game. Right or wrong, this is a very interesting theory you have here.