The following describes the truth of what happened behind-the-scenes in Alliance of the Golden Witch.
Culprit: Sayo Yasuda, dressed mostly as Shannon.
Accomplices: Genji Ronoue, Chiyo Kumasawa, Terumasa Nanjo, Toshiro Gohda, Jessica Ushiromiya, Kyrie Ushiromiya.
Before the murders: Sayo (probably dressed as Kanon) played the witch and met Maria at the garden. She decided to change her plans and told Maria to lie about having met Kinzo and that he gave her the umbrella. With this lie, Sayo lured all the adults to the dining room.
The first twilight (incompleted): Sayo reveals her true identity to all the adults as Lion Ushiromiya. Everyone was shocked but grudgingly accepted Sayo as their new head (hence they all accepted the existence of "Kinzo", which is technically another name for the "Head") after she shows them the ring. She then tells everyone that she is planning to surprise Battler with a murder mystery game. Everyone agrees, after she either promises them gold and riches or threatens them. With that, she tells Natsuhi, Rudolf, Hideyoshi, Rosa and Eva to stay in the dining room and not move an inch. She told Kumasawa and Gohda to run to the cousins and tell them that "something horrifying has happened" that is like magic to spook Battler out. She then leads Krauss, Nanjo and Kyrie into the basement dungeon leading to Kuwadorian. Essentially, nobody actually died—hence, an illusion. Only Kyrie knows that Sayo is planning to really kill everyone, while the rest still think it is a joke on Battler.
"Illusions to illusions. Tales woven by the gold truth return to illusions." - Willard H. Wright
The second twilight: Sayo told Krauss, Nanjo and Kyrie to call George and Jessica to do some kind of successorship test. She then dressed as Shannon to welcome George, and shot him in the rose arbor. She then dressed as Kanon to meet Jessica, forcing her to call Battler by telephone and tell him about the witch. She then killed Jessica.
"Illusions to illusions. Tales woven by the gold truth return to illusions." - Willard H. Wright
The fourth, sixth, seventh and eighth twilights: By this point, Shannon decided to "kill" off Kanon as a persona, and so he is considered dead by the narrative. Sayo goes to the dining hall where everyone is sitting unaware of the fact that a real murder is happening, and guns everyone down. Sayo and Kyrie kill Krauss and Nanjo, and Kyrie calls Battler to tell him about the witch. She then shoots at the door with her gun and drops the phone to pretend she's dead. Then Kyrie goes to meet Sayo to demand her payment. By then, however, Kyrie has already begun suspecting that Sayo is planning to kill her too, and tries to shoot Sayo. A scuffle occurs, with Kyrie getting killed by Sayo.
"Earth to earth. Illusions to illusions. Silent corpses, adorned by fiction." - Willard H. Wright
The ninth twilight: Sayo, with Genji's help, killed Kumasawa and Gohda by hanging them and shooting them in the head. She then puts fake tags on the wrong keys to make it seem like they were killed with the key being locked inside with them.
The first twilight (completed): Genji and Sayo returned to the dining room where she killed him, completing the first twilight.
The fifth and tenth twilights: Sayo poisoned Maria and moved her by Rosa's side at the dining room. By now, Sayo and Battler are the only ones alive on the island. Sayo, in her depression and fear of confronting Battler directly, took courage by drinking the remaining wine in Kinzo's study, then called Battler and started laughing. She asked him to meet her. She dresses herself in Beatrice's casual clothes, and dons a wig. She talks to him from the roof, and he fails to recognize her from the rain and the distance. She asks him if he remembers his sin. He does not, leaving Sayo completely and utterly crushed. She changes back to Shannon's clothes, and returned to the well where Nanjo and Krauss were left dead. Sayo then commits suicide by holding the gun under the well grate so that after she pulls the trigger it falls down the well.
Battler is left alone on the island. He inspected all the bodies, realizing that everyone on the island is dead and he has no way of leaving. At midnight, the bomb exploded, killing him.
"Earth to earth. Illusions to illusions. When fiction is shut up inside a cat box, it becomes truth." - Willard H. Wright