Dark Signs
Dark Signs is a case for the City of Mist roleplaying game by Son of Oak.
The players are assumed to play rifts that are interested in looking into abnormal phenomena.
To play you need at least the free Quick Start Rules . Works even better with the Players Guide¸ even so¸ it is not required and its companion the MC Toolkit.
This game contains mild language as well as strong themes of violence¸ crime¸ and immorality. Parental guidance is advised.
It is also making use of religious texts (mainly the Book of Revelation from the Bible). So if you or your players feel uneasy about it¸ either adjust the case or play another.