"It took me a long time to discover your weakness.... I thought it was pride, I thought it was the need to impress your father, but then I realised your weakness was the same as your father's - friendship."
-- Delphi (CC3.19)
Having taken them to the Quidditch pitch in the Hogwarts grounds, Delphi uses the Cruciatus Curse to torture Scorpius Malfoy in order to make Albus Potter follow her instructions. He and Scorpius need to humiliate Cedric Diggory again, this time during the Third Task of the Triwizard Tournament, so that Cedric again becomes the Death Eater who kills Neville Longbottom (CC3.19).
At the same time or just after the previous event, likely October 2020
This event takes place at the end of the Repaired timeline, as described in the essay "Albus Potter and the lesson in Quantum Mechanics" by Nick Moline.
Craig Bowker Jr becomes the second "spare" to be killed - a cruel and pointless death, like Cedric Diggory's (GF32, CC3.19).
Delphi also reveals the second prophecy involving Voldemort: "When spares are spared, when time is turned, when unseen children murder their fathers: then will the Dark Lord return". She wants to be at the side of the Lord Voldemort in a new dark and evil world. Craig was the latest spare and Albus will be the unseen child who will murder his father (CC3.19).
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, with Esther Smith in the role of Delphi, Sam Clemmett as Albus Potter, Anthony Boyle as Scorpius Malfoy and Jeremy Ang Jones as Craig Bowker Jr: Harry Potter and the Cursed Child, Parts One and Two