From Lezhin:
Past the grey desert and across the ocean lay the demon lands. Bloodthirsty and savage, they’ve waged war with the humans since anyone can remember. And seated on his throne, behind castle walls and royal guards, sits the demon king. At least, that’s what everyone thinks. And if Merlin ever wants to see his brother again, he’d better make sure everyone goes on believing he’s the demon king. Otherwise, who knows what the demons’ll do if they realize a human has been masquerading as their king?
Original Webcomic
Official English Translation