This was originally a web novel that was written by Tono Mamare, who also wrote Log Horizon.

We follow the hero as he enters the demon king’s castle and prepares to make the final blow, but before he does he listens to what the demon king has to say. The (quite feminine) king tells him how the war between humans and demons should continue because it’s better for both sides. She then tells him her plan to end the war peacefully and make the world a better place. So, he decides to work for her. The show is well made and has good writing, but if does feel a bit boring sometimes. Most of it is just talking about what’s going to happen when we do x and then we kind-of see x being done and the results of that.
The most entertaining parts are near the beginning, a brief period in the middle and then the end. The rest just feels a bit empty.
And as strange as it sounds, I feel like it may have progressed a bit too quickly. Near the beginning-middle we see 3 people getting educated by the demon king in like 2-3 episodes and around the end we see them again, but they feel like different people. It feels like a bit was left out where they undergo character development.

• aniDB
• theTVDB