"The question is whether it’s worth my time to kill you, knowing that as soon as I stop my father, your destruction will be assured. How to decide? Oh, I’m bored, I’ll kill you."
-- Delphi (CC4.11)
31 October 1981
While they wait for the transfigured Harry Potter to meet Delphi pretending to be her father and lure her inside St Jerome’s Church, Albus tells his mother Ginny that everything is his fault. Ginny replies that his father feels the same way “How funny. Your dad seems to think it’s all his. Strange pair, you are” (CC4.11).
The Voldemort disguise works at first and Delphi talks to her father. However, when the transfiguration starts to wear off, Harry is revealed. Delphi locks the others out of the room before they can intervene. She and Harry start duelling with magic (CC4.11).
While his father Harry is fighting her, Albus is able to get inside the church through a floor grate and unlocks the doors. The other adults are then able to join together with Harry to defeat Delphi (CC4.11).
All Hallows Eve - 31st October 1981
This event takes place during the Voldemort Final timeline, as described in the essay "Albus Potter and the lesson in Quantum Mechanics" by Nick Moline.
Initially, Delphi is stronger than Harry in the duel. She is able remove his wand very quickly with a voiceless incantation and says
"I’ve watched you for a long time, Harry Potter. I know you better than my father did." (CC4.11)
Harry knows he isn't stronger than her but once Albus opens the doors, they are together able to defeat her: "I’ve never fought alone, you see. And I never will." Although Delphi pleads to be killed, they refuse:
"Yes. Albus, she’s a murderer, and we’re not."
"We have to be better than them."
"Yeah, it’s annoying but it’s what we learnt."
-- Harry, Hermione and Ron (CC4.11)
From the Web
Pottermore: Harry Potter and the Cursed Child
The theatre production opened in the West End of London at the Palace Theatre on 31 July 2016: Harry Potter and the Cursed Child, Parts One and Two