Oaths of peace and wills of war (Deku x Apollyon)

What if the world didn't have quirks, what if their was a catastrophe event causing humanity to not advance for centuries and were stuck in their medieval times, what if groups from the past that never met each other were now at constant war, what if a certain stout and brave hearted boy joined the most warmongering legion of the knights and unknowingly stole the heart of his warlord.
This is the tale of a warden and warlords great battles, conquest, discovery's and love.