The House on Sorority Row

Seven sorority sisters pull a prank on their strict house mother, Mrs. Slater, who is known for carrying a sharp walking cane. Their plan is to put her cane out in their dirty pool and force her at gunpoint to retrieve it. The prank goes awry when Vicki, believing the gun to be loaded with blanks, shoots Slater who collapses and appears to be dead. The girls agree to hide the body in the pool until their graduation party is finished.
As the party begins, a guest wanders around the pool and an unidentified figure stabs him through the throat with Slater's cane. When other guests attempt to throw Jeanie in the pool, they are stopped by the sorority sisters. The girls realize that if the pool lights get switched on they will reveal Slater's body, so Stevie goes into the power room to disable them. While doing so, she is brutally stabbed to death by the same cane-wielding figure. Later, the pool lights do come on, but Slater's body is no longer in the pool. The girls speculate about what could have happened to both Stevie and the body.
Deciding that Slater must still be alive, Vicki orders the girls to search for her. Morgan enters Slater's room where Slater's trussed-up body falls on her from the attic. She is joined by the sisters, and Vickie suggests hiding the body in the old cemetery. Morgan runs crying to her room where she is stabbed to death by the killer with Slater's cane. Meanwhile, Katey goes into the attic and discovers children's toys, a clown statue, and a dead bloodied bird lying in its cage. Diane is waiting in her van for the other girls, but the murderer stabs her to death through the sunroof. Jeanie, the most nervous member of the group, is attacked while running back to the house, but manages to escape. After informing Katey about the attack, Jeanie arms herself with a chef's knife and is chased into the upstairs bathroom. As she hides in a stall, the killer enters the bathroom and looks in each stall. As the killer reaches the last stall, Jeanie bursts out of the stall to attack but the killer gains the upper hand and decapitates Jeanie with her own knife
Katey finds a medical alert tag on a necklace belonging to Slater. She calls the phone number and is put through to a Dr. Beck who soon arrives at the house. The two discover the bodies of Stevie, Morgan, and Diane in the pool before beginning to believe that Slater is responsible for the attacks. Meanwhile, Vicki and Liz are attempting to bury the body. As they are about to bury the body, Liz and Vicki are both shortly killed by the assailant. Katey and Dr. Beck travel to the cemetery where Katey finds the bodies of Vicki and Liz at the bottom of an open grave while Dr. Beck finds Slater's body in the back of the van.
After forcibly giving Katey a sedative upon returning to the house, Dr. Beck reveals that Slater had a son named Eric who was horribly deformed and mentally underdeveloped thanks to an illegal fertility treatment he had given her. Eric was living in the sorority house's attic and witnessed the death of his mother. He is now exacting revenge on the girls. Dr. Beck uses Katey as bait so he can capture Eric with a tranquilizer gun and cover up his crime. Upon seeing someone coming through the door, Dr. Beck shoots the person, only to see that it was Peter, Katey's date. Eric then arrives and hacks Dr. Beck to death while Katey searches for Vicki's gun. She discovers Eric standing over Dr. Beck's body, but when she pulls the trigger the gun will not fire. She flees to the upstairs bathroom to find and release the gun's safety catch, which she does, but panics when she finds Jeanie's severed head in the toilet. Katey climbs up into the attic where she is attacked by Eric, who is wearing a clown costume. Katey shoots him repeatedly, which reveals the gun is loaded with blanks. She then grabs a doll and uses the pin sticking out of its neck to stab Eric numerous times, and he falls through the attic hatch to the floor below. Katey peers down and is relieved to see him motionless, believing that he is dead. However, Eric was only stunned, as he opens his eyes before the film ends leaving both of their fates unknown.

Question: What does Eric witness?
Answer: the death of his mother