My former apprentice became the Demon Lord, so I was taken away by the hero.

In a world where the hero has defeated the Demon King and peace has returned to humans and demons, Shidan, a demon living a carefree life as a wandering painter, suddenly finds himself targeted by the Royal Knights. The crime Shidan is accused of is teaching martial arts to the Demon King, who has rapidly gained power and is causing riots in various places. Although attacked by the knights on suspicion, he overwhelms them with his rusty sword, demonstrating his strength that even the knight commander is no match for. Shidan longs for a peaceful life as a painter, but is placed in a "Covenant Ring" by the hero, who is in disguise, and is semi-forced to accompany him on a journey to defeat the Demon King...!?