"When spares are spared, when time is turned, when unseen children murder their fathers: then will the Dark Lord return."
-- the new prophecy (CC3.21)
Harry Potter, Draco Malfoy, Ginny Weasley, Hermione Granger and Ron Weasley make a search of Delphi’s room at the St Oswald’s Home for Old Witches and Wizards, trying to find out who she really is (CC3.21).
Ginny discovers a hissing oil lamp which Harry recognises as speaking in Parseltongue.
“I’m not supposed to be understanding – that’s Parseltongue…. I haven’t been able to understand Parseltongue since Voldemort died.”
— Harry Potter (CC3.21)
Using this to uncover the secrets within the room, they discover that Delphi is the daughter of Lord Voldemort. The new prophecy concerning the “unseen child” is also revealed (CC3.21).
Just after Harry and Draco speak to Amos Diggory at the St Oswald's Home, possibly at the same "time" that Albus, Scorpius and Delphi are in the past at the Third Task of the Triwizard Tournament - likely October 2020
This event takes place at the end of the Delphi Fails timeline and at the beginning of the Voldemort Final timeline, as described in the essay "Albus Potter and the lesson in Quantum Mechanics" by Nick Moline.
The cooperation between Draco, Harry, Hermione, Ron and Ginny continues to develop. Draco feels comfortable enough to make a joke about the spell he tries out (CC3.21).
Delphi's room has hidden writing all over the walls: the new prophecy and also the message "I will rebirth the Dark. I will bring my father back." We are left in no doubt that Delphi must have had evil intentions from the start. It is never clear how she knew something that was going to happen that involved the Diggory family - so she could be waiting at the St Oswald's Home (CC3.21).
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 Noma Dumezweni in the role of Hermione Granger, Paul Thornley as Ron Weasley, Jamie Parker as Harry Potter, Poppy Miller as Ginny Weasley and Alex Price as Draco Malfoy: Harry Potter and the Cursed Child, Parts One and Two
Harry Potter Wiki: Parseltongue