The Evil (1978) 
Overall, The Evil is a pretty formulaic haunted house flick in as much as we have a psychologist and his group of co-workers trapped inside an old mansion courtesy of a malevolent force of some kind.
Supernatural events start to occur gradually increasing in frequency and severity, in turn tormenting the various bell-bottom trouser sporting cast members, all of which leads to some creative kills and spills (and a spectral sexual assault) along the way.
However, the final act tilts more towards the bat-shit insane end of the paranormal scale, as our resident supernatural entity grows tired of playing little ghastly games in favour of revealing themselves and inflicting some truly torturous terror on the remaining members of the cast.
Evil by name, evil by nature indeed.