Delusion (1981)

Meredith Stone, a home care nurse, has been assigned the duty of looking after a wealthy invalid named Ivar Langrock. Cooped up in Langrock's stately country mansion, Meredith quickly suspects that someone sinister is lurking in the manor's dark halls only to discover that Ivar's demented and violent son has been locked away in a hidden room. Shortly thereafter, Ivar's grandson Gabriel also moves into the house and with his arrival a mysterious killer soon begins murdering members of the Langrock family along with their live-in staff.