The World's Finest Assassin Gets Reincarnated in Another World as an Aristocrat, Vol. 6 (light novel) - Reia Reia, Rui Tsukiyo

After Lugh’s most recent victory in his battle against the demons, he decides it’s time to take his relationships with Dia, Tarte, and Maha to the next level, proposing to all of them. The celebration is brief, however, for the Royal Knights Academy has finished rebuilding, and it’s time to return to school. Unfortunately, on Lugh’s very first day back, he’s given a new assassination target: the hierarch of Alamism. The religious leader has apparently been replaced by a demon in disguise. Lugh can’t let a monster control the world’s most powerful organization, but killing it will mean infiltrating the Holy Land, the most heavily guarded place in the kingdom. Has the assassin finally met his match?