Rodolphus Lestrange's prophecy

When spares are spared, when time is turned, when unseen children murder their fathers, will the dark lord return.

This prophecy was either made by or recited to Rodolphus Lestrange at some point between Harry potter and the Deathly Hallows and Harry Potter and the Cursed Child it prophesied the return of Lord Voldemort at the hands of Delphini.

Meaning
The first part When spares are spared refers to Cedric Diggory as just before Duel in Little Hangleton Lord Voldemort tells Wormtail to "Kill the spare."

The second part when time is turned refers to the Time-Turner used by Delphini to go back in time and attempt to save Cedric Diggory so that he will become a Death Eater and kill Neville Longbottom during the Battle of Hogwarts before Neville kills Nagini and subsequently destroys the last of Lord Voldemort's Horcruxes.

The third part when unseen children murder their fathers refers to Albus Severus Potter who embarrassed Cedric Diggory during the Second Task of the Triwizard Tournament causing Cedric to become a Death Eater who subsequently kills Neville Longbottom, this causes Voldemort to kill Harry Potter, Albus' father, therefor, through a series of events Albus kills his father. Albus is said to be unseen because his father doesn't understand him.

The last part is the result seen in the second alternate universe in Harry Potter and the Curse Child.