Otogi Taisen Phantasma

Otogi Taisen Phantasma (御伽大戦 ファンタズマ, Otogi Taisen Fantasma) is a manga written and illustrated by Hideaki Yoshimura. It was serialized in Comic Alive from March 27, 2013 to May 27, 2014.

Plot
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.