A Fairytale for the Demon Lord (마왕을위한 동화 -Der Erlkönig und der Ritter-) is a manhwa series by Kim Yong-Hwan. It was serialized in Daum Webtoon.
There once was a princess who fell for a curse. Countless times have a knight come in vain attempts to save the poor princess. They all failed, killed by the demon king. When a nameless knight finally comes to defeat the demon king, there appears an adventure never before predicted... A timeloop in which the nameless knight is fated to become either Odin or the demon king depending on his decisions.