Tensei shitara Slime Datta Ken: Coleus no Yume

Rimuru has saved the children he was teaching in the Kingdom of Engrassia, and his time as a teacher is nearly over. Then Yuuki, Grandmaster of the Free Union, asks him to get to the bottom of the race for the throne in the Kingdom of Coleus, which is located between Engrassia and the Holy Empire of Lubelius. So Rimuru infiltrates Coleus under the name "Satoru," and he finds a nation at the mercy of not only a battle for succession, but a web of schemes woven by demons and vampires!

(Source: Crunchyroll)