Fosvol was a Lux which inhabited a light bulb in the Nestdale Apartments basement. It was using the apartments to control the residents, using them as killers.
In the bodies of multiple residents, it killed off people from the neighborhood.
Once discovered, it stopped controlling some residents. The residents who were free overloaded the power from the apartments, sending thousands of volts of power into Fosvol.
Weak and with no body, Fosvol faded and died.