User blog comment:Dark Jeto/The Curse and the Dark/@comment-24709748-20140422034231/@comment-24679176-20140422202530

I think that the Undead Curse has somethig to do with the killing of the Everlasting dragons.

What was the main reason for Gwyn and friends to wage war against them? What evil did they do to them that they deserved complete anihilation?

They were everlasting, immortal beings and what is an undead? An everlasting, immortal being. Maybe, killing the dragons broke the balance in the world, maybe Seath, who coveted his brothers immortality, is the one to blame for the undead curse.

"So, you want to be immortal? Well, take it, but it will be an ugly and twisted kind of immortality that no one will ever want to have. Screw you all for killing us.

Yours truly. The Everlasting Dragons"

So, maybe the Undead Curse is a consequence of the first war with the dragons, some kind of final vengeance.