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

Meet the Evil King Valt, ruler of the Continent of Darkness. After being defeated by a group of heroes, he was reborn as a human, with his powers intact! "I'll start over in this world. And this time the whole world will be mine!" Valt garners the attention of the elite adventurer Iris. With Iris as his guardian, he enrolls at the Royal Adventurer Training School and immediately surpasses his seniors with his abnormal power! In an unprecedented move, this maverick is designated "level 0." He goes on to employ his talents as an Evil King to great effect, dominating the new world he finds himself in... (Source: SB Creative Corp., translated)