I'm the Villainess, So I'm Taming the Final Boss, Vol. 10 (light novel) — Sarasa Nagase

Claude receives a wedding invitation from his cousin Vika of the Kilvas Empire, and he and Aileen set out to attend. Not only is Vika the final boss of an otome game set in the same world -- he looks identical to the demon king. So when the groom goes missing, Claude somehow finds himself standing at the altar in his place! And amidst the turbulence of the empire, why is Claude's first love there?!