I have just watched the movie A Cure for Wellness. Can someone who is familiar with this movie explain to me what the doctor/baron did with his patients? I suppose that he in some way exploited them but how and why I completely missed.
Wikipedia has the answer
Lockhart investigates his suspicions and discovers the transfusion wing of the spa is a front for macabre medical experiments. The water from the local aquifer possesses unique properties. It is very toxic to humans, but to the eels living in the water, it has life restoring properties.
The baron had devised a process to filter the water through the bodies of humans, and distill the water into life giving essence. Volmer uses the patients as filters for this process. This "cure" is ingested by Volmer, Hannah and others in order to gain vastly lengthened lifespans.