Please Help Me Maohsama(2017)

Tasukete Maou-sama

Once upon a time, a fearsome demon king died at the hands of brave heroes, returning peace throughout the world. While the demon king themselves had passed away, traces of their power remain on the outskirts of the city. One of the heroes who slew the demon king continues on his quest to protect humanity, but what happens when a demon child gets attached to him?

(Source: Manga Planet)