Lily Potter and the Dungeon of Darkness
(Told in the next generation.)
The Wizarding World can finally live in peace, now that the Dark Lord Voldemort has been vanquished. Harry Potter, Ginny Potter, Ron Weasley, and Hermione Weasley don't have much to worry about. But their children do.
Hogwarts School of Witchcraft and Wizardry has changed, and only the Potters can sense it. The ghosts are becoming restless, and young Lily has been hearing whispering voices coming from the walls. Her siblings are trying to deny all of the strange events, and they don't believe their sister's wild stories. Lily must put it upon herself to investigate, even at the risk of her life.
Matters are worse outside of Hogwarts. The amount of magical blood has been decreasing drastically, and the Ministry of Magic have been noticing. Even pure-blood children are either Squibs or Muggles. Something out there is causing their powers to be diluted, some threat they aren't aware of. If this continues, witches and wizards may cease to exist. Their entire world is in danger.
I am not J.K. Rowling, which should be obvious. I own nothing but the plot. This is not to be associated with Harry Potter and the Cursed Child, as this is only a fanfictional story. Please don't copy my work. It's mine and mine alone.