Level 0 no Maou-sama, Isekai de Boukensha o Hajimemasu

Synopsis:

Ishuto was the mightiest Demon Lord of the Dark Continent until the heroes defeated him. But death wasn’t the end; he was reincarnated into a new world as a young man, still wielding his immense powers. Determined to build a life on his own terms, Ishuto declares, "This time, I’ll start from scratch and make this world mine!" The elite adventurer Airis, captivated by his strength, becomes his guardian so he can enroll in the adventurer training school. Yet, thanks to his otherworldly abilities, Ishuto graduates on the very first day, earning the unprecedented rank of "Level 0." A true maverick with Demon Lord powers unmatched in this world, his legend is just beginning...