we don't actually know that the abyss spreads because of darkness, and we DO know for a fact that the dark ending doesn't cure the darksign, neither ending does, and nobody really says it will. we know that darkness is part of the abyss, but lets look at what we know The Abyss spreads in Oolicile because they wake up Manus. Artorias goes in, and gets corrupted by it, saves Sif, and then you kill him. You stop Manus, and also stop the abyss by proxy. The Abyss DOSEN'T spread into New Londo. They flood it not to keep the Abyss at bay, but to seal the Darkwraiths and Four Kings. So, it seems, that the Abyss doesn't spread naturally, but Manus tries to spread it. There is also the fact that Artorias is corrupted, but not you. Artorias is not human, he's one of whatever kind of race the gods are. Meaning, he's more than likely a creature of light, much like Gwyn. You, however, are the only undead to set foot in the Abyss. You are not corrupted by it. This COULD mean that humanity is particularly resistant to dark, as humanity itself is parts of the Dark Soul. It could also just mean you never really spend a lot of time there. But once again, there's no real evidence that the Abyss normally spreads.
The Link The Fire ending is basically horrible and just a continuation of the cycle, but the Dark Lord ending is very ambiguous even with the DLC. You just don't have much clue what's going to happen. There are certain things that are true, such as that humans are affiliated with the dark, gods are affiliated with light, and saving one race kills the others. Otherwise, we don't have much to go on regarding exactly what happens. We're not even entirely sure that the Abyss is bad because of the darkness, or because of some other eldrich horror involved with it being a pit of infinite black. Personally I think that if the cycle is to continue, the Abyss would become the underground, while the above world would not become dark, but grey, like when the Dragons ruled. But that's just me.
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 ^it's a tumblr link oh geez^ oh man is this a steam profile
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