SERIOUSLY, SPOILER LAND.
I don't know if this has been posted here before, but someone over on /r/darksouls created a 5 part comic explaining the history (or lore) of the Dark Souls universe. Some of it is speculation, but whatever is speculation is pointed out. The little drawings are kinda cute, too.
I don't agree with some of it personally, but I love arguing with people over what some of this stuff means. Personally, I think the dark sign thing started when the furtive pygmy, Manus, was killed. The dark sign forces the undead to fight each other and hoard humanity until one eventually rises, and thus restores the Dark Lord. How that meshes with the fading of the flame, I'm not quite sure.
The current prevailing theory, and the one espoused in the comic, is that as humanity spread the number of dark soul fragments (humanity) increased until the undead thing started happening. How that meshes with the fading of the flame I'm also not sure.
One thing I didn't know is that it is implied that Eingyi may have started the problems with blight pus in Blighttown. I think I missed the item description leading to that one.
Anyway, I hope this is fun and informative to people who wanted to know more about the lore of Dark Souls without watching one of those 20 minute YouTube videos that sound like conspiracy theorists going off. Enjoy.