This is just a thought i have and maybe this wouldnt make sense lore wise but everytime i play through ds1 i think about it.
So the sealers of new Londo sealed the place up to keep the corruption of the abyss from spreading right?
Yet when you go there and unseal it the darkwraiths and dark blob things(whatever they're called) stay there.
Maybe it would be a cool idea for a mod where if you opened the seal early the game started replacing enemies with darkwraiths
over time while also making the games lighting darker in certain places. Just a thought...