Ancient Wizards Return to Hogwarts...

Salazar Slytherin was resurrected in the thousandth year after his burial.
After waking up, he changed his name and returned to Hogwarts as a new person, only to be shocked to discover:
Families that were keen on marrying Muggles or were themselves Muggle-born claimed to be pure-blood families.
Out of nowhere, a madman pretending to be his descendant became a fearsome figure known as the Dark Lord, spreading extreme ideas under his own banner.
As spells became increasingly elaborate and complex, wizards' bodies became unusually weak, even the descendants of Gryffindor.
Slytherin: Voldemort, I think you're going a bit to the extreme—and how come I didn't know I built a secret chamber in the castle? Wait, you're not referring to that basement I use for teaching, are you?!