I’m the Villainess, So I’m Taming the Final Boss, Vol. 4 (light novel)

Holy King Vs. Demon King!
Aileen had set off on what was supposed to be a diplomatic mission to improve relations with the Kingdom of Ashmael and its Holy King—so how did she end up in his harem?! What’s worse, Holy King Ashmael is the final boss from the third game in the series, and he’s immune to Claude’s magic. Now that it’s come to this, is Aileen’s only choice to romance the Holy King?