I was reading this manhwa recently called A Fairytale For The Demon Lord and was surprised at the quality of the art for a weekly release.
It actually started as a webcomic but it turned to be longer than originally planned. Although its not groundbreaking story, its nice to read a knight saves the princess story (even though the princess is super strong in this case). It's a very new manhwa so the plot hasn't developed much but it already gives some hints.
The synopsis
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 a demon. When a knight finally comes to defeat the demon, there appears an adventure never before predicted...
The original site for the web comic can be found here.
I think the nameless Knight might be the demon lord since something to do with time was mentioned in the first few chapters. That or the nameless knight will become the new demon lord - they can cast the same spells and hold the same feeling of affection to the princess.