Thir13en Ghosts

Ghost hunter Cyrus Kriticos (F. Murray Abraham) and his psychic assistant Dennis Rafkin (Matthew Lillard) lead a team on a mission to capture a spirit called the Juggernaut (John deSantis). Several men are killed, apparently including Cyrus. However, the team is able to catch the ghost. Cyrus's nephew Arthur (Tony Shalhoub), a widower, is informed by Cyrus's estate lawyer, Ben Moss (J. R. Bourne), that he has inherited Cyrus' mansion. Financially insecure, Arthur decides to move there with his two children, Kathy (Shannon Elizabeth) and Bobby (Alec Roberts) and their nanny Maggie (Rah Digga).
Posing as a power company inspector, Dennis meets the family and Moss as they tour the mansion. The residence is made entirely of glass sheets inscribed with Latin phrases, which Dennis recognizes as barrier spells. While searching the basement, Dennis is hit by psychic flashes and discovers the twelve angry ghosts he and Cyrus captured are imprisoned in the house, held captive by the spells. As Dennis warns Arthur, Moss slips downstairs and picks up a valise of cash, unwittingly triggering a mechanism that seals the house and releases the ghosts one by one. He dies when a set of sliding doors snaps shut, cutting him in half. Bobby slips away from Kathy and Maggie and enters the basement, where he sees several of the ghosts, including that of the Withered Lover - his mother Jean, who had died of injuries sustained in a house fire. He is knocked unconscious and dragged away.
Using a pair of spectral glasses that allow the wearer to see into the supernatural realm, Dennis convinces Maggie that the ghosts are real. Dennis discovers that the Jackal, one of the most dangerous of the twelve ghosts, has been released and that the family is now in grave danger. The Jackal attacks Kathy when she and Arthur enter the basement in search of Bobby, but they are saved by Kalina Oretzia (Embeth Davidtz), a spirit liberator who is attempting to free the ghosts. Kathy disappears soon afterward, and the four adults gather in the library, where Arthur learns that Jean's spirit is trapped in the house. Kalina explains that the house is a machine, powered by the captive ghosts, that can allow its user to see the past, present, and future. The only way to shut it down, she says, is through the creation of a thirteenth ghost from a sacrifice of pure love. Arthur realizes that he must become that ghost by dying to save his children.
Armed with a pane of the special glass, Arthur and Dennis enter the basement to find the children. Dennis barricades Arthur into a corner behind the glass and allows two ghosts, the Hammer and the Juggernaut, to beat and kill him. Cyrus faked his death to lure Arthur to the house; Kalina is his secret partner. Cyrus has orchestrated the abduction of Kathy and Bobby so that Arthur will become the thirteenth ghost, which will not stop the machine as Kalina had claimed, but trigger its activation. Cyrus kills Kalina and summons the ghosts to activate the machine.
Arthur arrives at the main hall and witnesses all twelve ghosts orbiting a clockwork device of rotating metal rings, with his children at the center. Discovering Cyrus' true fate, Arthur fights Cyrus while Maggie disrupts the machine's controls. The ghosts hurl Cyrus into the rings, slicing him to pieces. With the encouragement of Dennis' ghost, Arthur jumps into the machine, avoiding the rings and saving his children. The walls of the house shatter as the malfunctioning machine rips itself apart, freeing the ghosts. Dennis smiles at Arthur and departs, and Jean's ghost appears before the family and tells them that she loves them before she and all the other ghosts disappear.
As the family leaves the house, Maggie exclaims that she is quitting.

Question: What is the terrible discovery that Arthur makes?
Answer: Arthur realizes that he must become that ghost by dying to save his children.