|"Professor McGonagall told me what awful things have happened when wizards have meddled with time…"|
- "I don't fight alone, and I never will."
- — Harry Potter to Delphini[src]
Travelling through time
- "I am the new past. I am the new future. I am the answer this world has been looking for."
- — Delphini snaps Albus Potter's and Scorpius Malfoy's wands[src]
Delphini was born in secret at Malfoy Manor to Bellatrix Lestrange and Lord Voldemort. Both of her parents were killed in the Battle of Hogwarts, leaving her to be raised by Euphemia Rowle. Eventually, Rodolphus Lestrange told Delphi about her true heritage and the prophecy he believed she was meant to fulfil.
On 31 August, 2020, Amos Diggory went to Harry Potter and urged him to use a Time-Turner to go back in time to the Triwizard Tournament and save his son, Cedric, who was murdered on the orders of Voldemort after winning the event. Harry refused, but his son, Albus, overheard the conversation. Later, along with his friend, Scorpius Malfoy, he decided to help the Diggorys to alter history and save Cedric. The two found their way to St Oswald's Home for Old Witches and Wizards, the nursing home where Amos Diggory lived and where Delphi, posing as his niece, pretended to take care of him.
Albus, Scorpius, and Delphi sneaked into the Ministry of Magic and stole a Time-Turner. Using the Time-Turner, Albus and Scorpius seriously altered past events, creating two alternative realities, bringing about catastrophic results. After many attempts, the correct timeline was restored. After Scorpius decided to destroy the Time-Turner once and for all, Albus sent an owl to Delphi. She arrived to Hogwarts, where she suggested they should destroy the Time-Turner together, and that she would explain to her uncle why Cedric would not be saved.
After Albus handed her the Time-Turner, both boys noticed a black tattoo of an Augurey on her back. The boys realised she was an enemy based on Scorpius recalling references to 'the Augurey' in one of the warped timelines, but Delphi pulled out a wand and bound them together. Delphi planned to travel back in time to ensure Cedric's survival in the third task of the Tournament. Craig Bowker Junior, another Hogwarts student, rushed up to say that the whole school was looking for them, but Delphi struck him with a Killing Curse. Delphi took Albus and Scorpius back in time with her, but they refused to cooperate. As Delphi prepared to kill Scorpius, she was struck from behind by an unsuspecting Cedric Diggory, who thought the scene was simply a part of the Triwizard task. He freed the boys before returning to the challenge. Taking advantage of her incapacitation, the boys raced through time once again, but once they arrived, Delphi destroyed the Time-Turner and flew away. Albus and Scorpius discovered that they had been transported to 30 October 1981 — the day before Harry Potter’s parents were murdered by Voldemort. The boys realised that Delphi intended to stop Voldemort from trying to kill Harry, and thus triggering his own destruction when his Killing Curse rebounded on him due to Lily Potter's loving sacrifice.
Meanwhile, the boys' parents were looking for them. Ron Weasley told Harry he had seen Albus with an older girl the night before. Harry thought he must mean Delphi Diggory, Amos’s niece. They went to see Amos, who told them that he never had a niece. The adults raced to Delphi’s room and discovered writings revealing Delphi to be Tom Riddle's daughter and that she believed she could restore him to power following the words of a prophecy.
The Potters' arrival
Albus and Scorpius were able to send a message to their parents by leaving a message on Harry's old baby blanket, treated with a potion that would only become visible when it interacted with another potion Albus had spilled on the blanket shortly before this final trip back through time. Now aware of when and where their children were, Harry, Ginny, Draco, Ron, and Hermione headed back to Godric’s Hollow in 1981 using another Time-Turner in order to help the boys. They found Albus and Scorpius, and all of them hid in a church, from which they could look out for Delphi without being seen.
Delphi finally appeared and Harry (disguised as Voldemort) pretended not to know who she was. She explained that she was the child of Tom Riddle and Bellatrix Lestrange. He told her that together they could become an unstoppable force, but, by that time, the transfiguration had begun to fade.
A magical showdown
- "You can't remake your life. You'll always be an orphan. That never leaves you."
- — Harry Potter's words to Delphi about her father[src]
Delphi saw that she was actually speaking to Harry, claiming she had studied him and knew him better than her father did. She wandlessly locked the doors to prevent the others jumping out to help. Without a wand, Harry was forced to hide beneath a church pew while she attacked him. When Delphi was about to kill him, Albus popped up through a hatch in the floor, throwing a wand to his father. Delphi still overwhelmed Harry, but then Ginny, Draco, Ron, and Hermione converged on the scene. Together, they were able to overpower Delphi.
Delphi begged them to kill her or wipe her mind clean. She claimed that she had only wanted to meet her father and be with him. Albus wanted to kill her to avenge Craig, but Harry stopped him. In a moment of empathy for her, Harry told her that she must learn to live with the fact that she will always be an orphan, as he has.