Every Harry Potter book has at least one, with most of them having to do with the villain's identity and true nature. As Cerebus Syndrome set in, the series gradually came to deemphasize them, with the climax of each book instead revolving around the death of a major character; even so, they're still very much present in the later books.
- Philosopher's/Sorcerer's Stone: Professor Quirrell is the traitor at Hogwarts trying to steal the Sorcerer's Stone, and Voldemort has been possessing his body all along.
- Chamber of Secrets:
- The Heir of Slytherin is Tom Riddle—better known as Lord Voldemort. Ginny Weasley has been his Unwitting Pawn, as she opened the Chamber while possessed by the spirit of Riddle's old diary.
- Harry is actually a Parselmouth, who could talk to snakes, and the reason it is important because Voldemort was the most famous recent speaker of it.
- Prisoner of Azkaban:
- Moony, Wormtail, Padfoot and Prongs are Remus Lupin, Peter Pettigrew, Sirius Black and James Potter, respectively.
- Sirius Black is innocent, and he was loyal to the Potters until the end. The one who actually betrayed the Potters to Voldemort and commited the murders that got Black sent to Azkaban was Peter Pettigrew, a.k.a. Wormtail.
- Pettigrew is still alive, and he has spent the last 12 years posing as the Weasley family's pet rat, Scabbers. Black has been trying to break into Hogwarts to kill Pettigrew the whole time, not Harry.
- Professor Lupin is secretly a werewolf. Snape has been trying to expose Lupin as a werewolf to get him fired from his teaching post at Hogwarts.
- The Shrieking Shack is actually a sanctuary that Dumbledore built for Lupin when he was a student at Hogwarts, and still trying to cope with his lycanthropy. The Whomping Willow was planted to hide the entrance to the secret tunnel that leads into the Shack.
- The mysterious Black Dog was Sirius Black the whole time, as he's been looking after Harry while in his Animagus disguise.
- Hermione has been keeping up with her class schedule with the help of a Time-Turner, a powerful magical device that lets her make short trips back in time.
- Goblet of Fire:
- Alastor "Mad-Eye" Moody is really Barty Crouch Jr., who has been hiding his identity with Polyjuice Potion ever since escaping from Azkaban.
- The Triwizard Tournament is secretly a trap designed to deliver Harry into Voldemort's hands; Crouch/Moody arranged for Harry to enter the Tournament so that he could help him win, ensuring that Voldemort could use his blood in his resurrection ritual.
- Order of the Phoenix:
- When Umbridge is interrogating Harry on why he was using her Floo Network to try to contact Sirius, she reveals that she was the one to send the Dementors to attack Harry at the beginning of the book, not Voldemort like everyone had believed up until that point. She did this to force Harry into a position of either being removed from the Ministry's hand by Dementor's Kiss, or get him expelled and get him out of their hands in a less lethal way.
- The mysterious hallway from Harry's dreams is the Ministry of Magic's Department of Mysteries, which houses a prophecy about Voldemort's defeat at Harry's hands—which first led Voldemort to target Harry as a baby. Harry has been dreaming about the hallway because Voldemort projected visions into Harry's mind to trick him into retrieving the prophecy so that he could steal it.
- Half-Blood Prince:
- The Half-Blood Prince is Professor Snape—the Half-Blood son of a woman named "Eileen Prince".
- Professor Slughorn was the one who told Tom Riddle that crafting multiple Horcruxes is possible when he was a student at Hogwarts.
- Deathly Hallows:
- "R.A.B." is Sirius's deceased brother, "Regulus Arcturus Black".
- The Invisibility Cloak is really one of three fabled magical items—allegedly once gifts from Death—that can supposedly make the owner immortal if they're all found.
- Professor Dumbledore was once close friends with Gellert Grindelwald, the Dark Wizard who he's credited with defeating. He was responsible for taking the Elder Wand from Grindelwald in single combat, making him its most recent owner.
- Professor Snape was on Dumbledore's side the whole time, as he defected from the Death Eaters to avenge the murder of Lily Potter—the only woman he ever loved. Dumbledore actually ordered Snape to kill him, because he knew that he was dying from the curse placed on Marvolo Gaunt's ring, and he wanted to die on his own terms to prevent Voldemort from taking the Elder Wand. Snape was the one who cast the doe Patronus to guide Harry to the Sword of Gryffindor.
- Dumbledore's old Snitch contains the Resurrection Stone, and it's designed to open when Harry announces that he's about to die.
- The final Horcrux is Harry himself. Voldemort inadvertently left a fragment of his own soul in Harry's body the night that he murdered Lily and James.
- The rightful owner of the Elder Wand was really Draco Malfoy, who disarmed Dumbledore before Snape killed him. Professor Snape didn't really claim the Wand from Dumbledore, because Dumbledore chose to die. Harry unwittingly won possession of the Wand when he disarmed Malfoy in single combat at Malfoy Manor.
