A Fairytale for the Demon Lord ~ Kim Yong-hwan
You can read it on: http://www.mangareader.net/a-fairyta...the-demon-lord
From: Easy Going Scans
A Demon Lord spirited the Princess away, and cursed her with eternal sleep. Countless knights sought to save her, only to die in vain by the Demon Lord’s hands. But then, a Nameless Knight arrived to vanquish him and save the Princess, ignorant of what fate had in store for him...
Genre(s): Adventure, Fantasy, Romance, Seinen, Tragedy, Webtoons
It's actually an Korean Manhwa Webtoon, but it's categorized as a seinen, so here it is ;). This story might be pretty confusing, but it's worth the read, not to mention the art is really good and there might be a sequel to it. It bases on Norse mythology, but don't look too much into it ;P. I think that this is pretty complete, unless the second part (if it appears) will be under the same name. Anyway, enjoy ;).