Seidou no Alice

A stranger appears in frontier lands.
Its true nature: a demon; an incubus.

Seeking to drive off this darkness,
the church summons Sharon, a woman trained in the Way of Sex.
However, ten nights after leaving on her mission, she had still not returned.
Her successor, Alice, discovers the incubus has infiltrated a brothel.
She passes herself off as a prostitute and awaits the demon.
Her unease swells as she waits.
Then, a man with red eyes appears.

[From DLsite English]