From publisher blurb:
Oni Palace is a Fifth Edition plug-in adventure for four characters with an average party level (APL) of 5, 8, 11, or 14. This addon offers a general guideline on how to scale the adventure for each level. The characters infiltrate a martial arts sect to confront its leader only to realize that this leader has been replaced by a dangerous oni.
An oni with a taste for powerful mortals has taken control of the Zandougong sect, a prestigious martial arts group located deep within a secluded mountain range. Safe within his heavily guarded palace, he only interacts with other members of the sect, each a powerful warrior who has proven themself through trials and combat. None of the members of the sect know that their leader was replaced by an oni a few years ago and are fanatically loyal to him.