Black Christmas

William Edward "Billy" Lenz, a boy born with severe jaundice due to a liver disease, is constantly abused by his hateful mother. After murdering Billy's father and burying his body in the underground crawlspace with the help of her boyfriend, Mrs. Lenz locks Billy in the attic to prevent him from talking. Years later, she attempts to conceive a new baby but realizes that her new man is impotent. She goes up to the attic and rapes 12-year-old Billy. Nine months later, a daughter named Agnes is born and treated like a princess by Mrs. Lenz. When Agnes is eight and Christmas comes around, Billy escapes from the attic and disfigures Agnes by gouging out her eye. He then taunts his terrified mother, saying "she's my family now," and gruesomely kills his mother and her lover. He is caught by the police eating cookies made out of his mother's flesh, and is sent to a mental asylum.
15 years later, on Christmas Eve, Billy, now 35, escapes from his cell and heads off to his former home, now a sorority house. At the Delta Alpha Kappa, Clair (Leela Savasta) is killed in her bedroom. Meanwhile, Megan (Jessica Harmon) begins to hear noises and goes up to the attic to investigate. Upon finding Clair's body in a rocking chair, Megan is attacked and killed. In the living room, the other sorority girls receive a call from a rambling man, who ends the call threatening to kill them.
Clair's half-sister Leigh (Kristen Cloke) soon arrives searching for her. When the lights suddenly go out, Dana (Lacey Chabert) goes to check the powers under the house, but encounters the figure and is killed with a gardening tool. The girls in the house receive a call from Dana, and hear a scream. They leave the house to find her, only for Kelli (Katie Cassidy) and Melissa (Michelle Trachtenberg) to discover blood splatters under the house, while Heather (Mary Elizabeth Winstead) and Leigh find Eve (Kathleen Kole) decapitated in her car.
Heather and housemother Mrs. Mac (Andrea Martin) immediately flee, but Heather is subsequently killed while inside the car and Mrs. Mac is stabbed through the head by a falling icicle. When Kelli and Leigh descend to the garage to investigate, Melissa is attacked and killed. Kelli and Leigh then find Lauren (Crystal Lowe) with her eyes gouged out in bed. Kelli's boyfriend Kyle (Oliver Hudson) then arrives, claiming he is not the killer. The three climb to the attic, where Kyle is dragged and stabbed in the head. The killer is revealed to be Agnes, now an adult. Billy, revealed to be a second killer, also makes his way into the attic and both killers close in on Kelli and Leigh, starting a fire. Kelli and Leigh manage to escape and leave Billy and Agnes to burn in the fire.
Later, Kelli and Leigh recover at the hospital. While Kelli goes for an X-ray, Agnes appears in the hospital unharmed and kills Leigh by snapping her neck. When Kelli returns to her room, Agnes enters through the ceiling and attacks her as well but Kelli uses the defibrillator and kills Agnes. Moments later, however, Billy immediately enters through the ceiling as well and chases Kelli to the stair-rail, where Kelli ends up pushing Billy off the stair-rail where he is impaled on the tip of a Christmas tree, finally killing him, and Kelli is left to look in shock.

Question: What is Mrs. Mac stabbed through the head by?
Answer: falling icicle