I have an extremely broken recollection of a horror movie I've watched many years ago. I watched a Russian dub of that film. I'm subscribed to a TV provider known as Kartina TV, and one of the services that it provides is Russian dubs of films that were originally in some other language. I have no idea what the original language of that film was.
A group of people for some reason travel to an abandoned monastery or insane asylum (I don't remember which one it was). After a while, one or more of the visitors stumble across a room where they learn that, before it was closed, the place was used for experiments on humans where the patients were injected with some toxin from some fly, with the goal of "Transforming the brain into a sort of reproductive organ" (That's the wording that the dub I watched used, the wording in the original film might be different), but which resulted in them becoming monstrous creatures. After that, one of the patients, patient one, appears and attempts to subject one of the characters to the procedure.