Return to House on Haunted Hill

Ariel Wolfe (Amanda Righetti) is the sister of Sara Wolfe (Ali Larter), a survivor of a birthday event eight years ago in the Vannacutt Psychiatric Institute for the Criminally Insane. In the 1920s, the asylum was overseen by the sadistic psychiatrist Dr. Richard B. Vannacutt (Jeffrey Combs). Sara claimed that ghosts of the house residents killed the party guests, and later commits suicide.
Ariel and her friend Paul (Tom Riley) are kidnapped by an art dealer, Desmond Niles (Erik Palladino). Ariel realises that Sara didn't commit suicide: Desmond killed her. Desmond forces Ariel to help him find an artifact located inside the old Vannacutt Psychiatric Institute, a figurine of the demon Baphomet. Inside the building, they encounter Dr. Richard Hammer (Steven Pacey) and his assistants Kyle (Andrew-Lee Potts) and Michelle (Cerina Vincent).
Ariel explains that the building has been rigged to keep everyone inside for at least 12 hours. The group splits up to search for the idol. Desmond's henchmen are killed by the inmate ghosts, having visions of the patients there suffering the same deaths as them. A ghost shows Ariel the depravity the inmates suffered under Dr. Vannacutt. These images reveal that Vannacutt was driven mad by the idol, and performed experiments on the mentally ill. The inmates led a revolt against Vannacutt, during which the sanatorium burned down. (The audience is shown footage from the 1999 film House on Haunted Hill.) The deaths in the previous film were assumed to be caused by the ghosts. But now Ariel is shown that the dead are actually forced by the idol to do Vannacutt's bidding and did not willingly kill.
Although the 12 hours are up, the master locking mechanism begins to lock the house down again. Ariel escapes but discovers that Paul has entered into the house to look for her, and goes back inside. Convinced Michelle wants the idol for herself, Desmond attempts to kill her. Michelle however is killed by Vannacutt. The rest of the group discovers a way out of the asylum but it is blocked by iron bars. The ghost of an inmate shows Ariel that the idol is in the asylum's basement crematorium.
Ariel, Paul, and Dr. Hammer descend to the crematorium and discover the "heart of the house," composed of living flesh. Ariel tries to destroy the idol but it is indestructible. She then reasons that if it is flushed down the sewer and leaves the building, the spirits will be freed. The team is ambushed by Desmond, who wants the idol. The ghosts seize Desmond and burn him alive after he has a vision about a patient dying a similar death. Dr. Hammer is overcome by the idol's evil and tries to strangle Ariel. The ghost of Vannacutt and inmates appear, Vannacutt hoping one of them will die in the fight. Hammer recovers his senses, but Dr. Vannacutt kills him. Ariel throws the idol into the sewer. The spirits vanish, and several attack Dr. Vannacutt, tearing him apart. The building comes unsealed and Ariel and Paul leave.
In a post-credits scene, a man and woman are about to have sex on a beach. The woman feels something under the sand. They dig, and pull the Baphomet idol into the light.

Question: The spirits vanish when Ariel throws the idol where?
Answer: Into the sewer.