Otogi Taisen Phantasma

Yamada Tarou is living a regular life, until the death of the father he never sees catapults him into the role of the 13th King of the holy Phantasma kingdom. Now that he's king, Tarou has become the target of assassins, and all he has on his side is a mistress, Princess Kaguya, whose goal is to make Tarou a worthy king. (Source: MU)