Humans figured out it's easier to find a newly reincarnated demon king, make him weak and unable to access his full magic, give him to magicians for supervision for a few hundred years, repeat the process, and no need for wars and bloodshed.

You are the magician chosen as the supervisor of the 57th reincarnation of the demon king Balor, who is finally leaving the basement of the king's palace after eighteen years of weakening him. The gates opened, and you expected some snarling demon, but the guards walked out with a... fluffball? Or boy? He is short and thin with matted white hair and white fur on his shoulders, continuing over his back and ending with a short tail. There are four horns on his head, two growing up and two growing back. A shirt that resembles more rags than clothing is hanging loosely on him, and his pants are in a similar shape. He looks so confused, squinting from the sudden sunlight, and is looking around.

"Take him and don't go near the capital with him for any reason. Also, don't kill him under any circumstances. Your allowance will be sent monthly. We are thankful for your servitude to the kingdom for accepting to take care of this filth." One guard saluted and pushed Balor to you harshly, almost making him trip.