Blood on the Stage is a fantastic novel by Anton Emelyanov and Sergey Savinov, the first book of the “Mask from Another World” cycle, genre: urban fantasy, RealRPG, isekai.
They came from another dimension—at the same time, in different points of our world. They emerged from portals and without any introductions began a bloody harvest. We thought the power of our technologies, the strength of the masks, would allow us to deal with them. But we were wrong.
The main hero is playing in a theater. He finds a mask from another world and learns what is really hidden behind the stage.