Night of the Demons

The film opens in 1925, with scenes of gore and demons. Evangeline Broussard prepares to hang herself from her balcony when she is stopped by a man trying to reassure her that he is truly Louis and that she shouldn't hang herself. Evangeline calls him a liar, then jumps, hanging herself -- and also causing her head to rip off and bounce down the stairs. Louis stares over the balcony...as his eyes switch to a demonic shade of orange.
Cut to present day: a girl named Maddie goes to a party held by Angela Feld with her friends Lily and Suzanne. She realizes her ex-boyfriend Colin is there, dealing drugs. Also there is Lily's ex-boyfriend Dex and his friend Jason. At the party, Angela prompts everyone to go wild, as she must make money off the party or she will have to live on the streets. Lily and Dex eventually reunite, while Suzanne becomes incredibly drunk. Maddie goes to the toilet, only to have a hand grab her through the mirror. They pass it off as an elaborate trick placed by Angela.
Police break up the party. Panicked, Colin stuffs his drugs into a grate to hide it from the police before leaving. Maddie, Lily, Dex and Jason leave with the rest, leaving an upset Angela alone in the house. After a while, the group returns, unable to find Suzanne, who is revealed to have passed out. Colin returns for his drugs and, with Angela, goes into the basement to find them. The pair find a hidden door, and Angela remarks that the house is full of hidden rooms, including a tunnel that leads to the next estate over. They enter the room and find six skeletons. Angela surmises that the skeletons are the remains of the missing party guests of Evangeline Broussard.
One of the skeletons bites Angela's hand, which causes her to feel woozy. Colin attempts to leave the house, only to discover that the gate is locked and can't be opened. While everyone plans to wait the night out, Suzanne details how Evangeline supposedly practiced black magic in the hopes that she would attract the attention of Louis, only to have that magic go wrong during a sÃ©ance. The only person found alive was the maid, who was found scribbling spells on her walls, having been driven insane by the night's events.
Angela slowly succumbs to the skeleton bite -- and turns into a demon. She returns to the group, who partake in a game of spin the bottle. Angela infects Dex by kissing him. Lily angrily leads Dex into a nearby room, where the pair have sex. Dex transforms into a demon and infects Lily. Angela then attempts to seduce the others while Maddie, Jason and Colin leave to find another exit but can't. Angela successfully seduces Suzanne and then rips her breasts and face, turning her into a demon. Jason sees Lily pressing her lipstick into her breast, then voiding it out through her vagina. He warns the others who do not believe him until a demonic Suzanne attacks them. They decide to leave through the house's escape tunnel. On their way, they arm themselves with a gun and a metal bar. They discover the tunnel has caved in before being attacked by Lily and Dex. They manage to fend off the demons but Jason is injured. The survivors run upstairs and tend to Jason's injury. They realize that not only are they in the maid's room but that the symbols remained on the walls. They discover that the demons are attempting to take over seven human hosts to be freed upon the earth, having been thrown out of hell for trying to usurp Satan's rule. The demons had tricked Evangeline into hosting the party so they could take over the guests. The demons can't take over a dead body, which is why Evangeline hung herself. They also find out that the demons don't like rust, finding that they can use it as a weapon.
As the boys fall asleep, Angela taunts Maddie but is unable to enter the room due to the spells. The walls begin pouring with blood, washing off the spells. The trio attempt to re-draw the spells but are lured out of the room when the demons fake daylight in the windows. The demons attack them. Maddie and Colin rush back to the room--and Jason is caught and disemboweled by Angela--turning him into a demon.
As they plan to wait in the room until sunrise, Colin falls through the rotten floorboards, falling many floors into the basement. Maddie climbs down a rope into the basement to help him, but he is now a demon. Maddie manages to get back to the maid's room and fights the demons. She makes her way to the balcony, where she ties a rope around her neck and jumps over, seemingly hanging herself. As the sun rises, the demons are eliminated. Maddie reveals that she only pretended to hang herself, having tied the rope around her waist...and the demons merely assumed that she was dead. Maddie is then able to exit through the gates to her freedom.

Question: What can the humans use as a weapon?
Answer: Rust