Hell Night (1981)

Four fraternity and sorority pledges must stay over night in a possibly haunted mansion.  The initiation takes place on the 12th anniversary of a mass murder, where a man killed everyone in his family except the family gork, which apparently is their term for mentally retarded psychotic mutant.

After the killings were discovered, the police could find no signs of the gork or the three bodies. Legend has it that the gork still lives in the deserted mansion.

The head of the frat padlocks the gate behind the pledges, who  conveniently are two boys and two girls.  The loose girl and guy head upstairs to satisfy their lustful desires.  The chaste couple sit downstairs and talk innocently until they hear a horrifying scream from upstairs.  As the couples react in fear, we are privy to a scene of the frat president snickering and holding a microphone attached to an elaborate group of wires running inside the mansion to a speaker system.

When both couples investigate, they meet up and realize the scream wasn't one of them.  After a search, they find a speaker hidden behind a drape and disable it.  I was impressed that they not only found the speaker,  but also rendered it useless since most horror flicks would have the kids running in circles screaming before accidentally dismembering themselves on an old suit of armor holding an ax.

Turns out the frat rigged the house for the initiation, complete with dummy corpses and projected apparitions.  Also it's the night of the frat costume party.  So if you see anyone in a gorilla mask, or a strange disguise it is explained away  That also makes it easier for a killer to walk among them.  So is there really a mutant psycho killer living in the mansion, or is this all a prank?

The answer becomes obvious once the frat boys tormenting the pledges get killed by a man with large dirty hands. From the size and filthiness of the hands, perhaps this is a psycho retarded mutant killer?  Of course it is!  Who else is going to kill frat kids preforming an initiation at the ancestral home where their father murdered their entire family?

Hell Night is classic 80s horror - the frat, the deranged mutant killer, the abandoned mansion, the pranks on the pledges, and when the police are notified of the killings, they won't believe it due to the  frat's propensity for hijinks.  Plus it has Vince Van Patten and Linda Blair.