Youmu is famously half-human and half-phantom. In this "possessed" state, you can toggle between two combat stances that drain a "Sanity Meter" Human Stance (Roukanken):
" (Possessed by a Demon) suggests a dark, horror-action hybrid or a "corruption" themed gameplay style common in the 2021-era doujin scene. youmuinthe nightmaretaker akuma ni tsukareta 2021