Is This The Most Powerful Tool In Harry Potter's Wizarding World?
A fantasy universe is only as good as the rhetorical nerd fights that it spawns. George Lucas would've made 12 whole dollars if he'd released a movie called Star Peace or used lightsabers as talking sticks. People want to see powerful, fictional, highly merchandisable artifacts getting smashed into one another, and they want to know which one is the best.
In the Harry Potter universe, this presents a problem. Right off the bat, the series offers readers a world populated by children waving literal magic wands, capable of doing just about anything. Want to capture a shade-like representation of a dead friend or relative and hold it indefinitely in a painting? You're a $30 learning annex course away from being able to do just that. Want to kill somebody? They teach you all two of the words you need to know ... in school, when you're about 14 ... then make everyone promise to be cool about it.
Still, there are a few remarkable relics in the Wizarding World that go above and beyond, pulling more than their magical weight. The Felix Felicis potion has the power to turn whoever imbibes it into a supernatural Mister Magoo, stumbling grinningly towards success, while the Philosopher's/Sorcerer's Stone made folks immortal, apparently without any side effects. But for fans' money, there's one tool in J.K. Rowling's stories that beats them all — one for which Rowling doesn't even get to take full credit.
Time travel complicates everything, even in Harry Potter
Longtime fans will recall that the third entry in the series, Harry Potter and the Prisoner of Azkaban, introduces a lot of new details to the universe. Not the least of these is the Time-Turner, a seemingly all-powerful piece of enchanted jewelry. The gyroscopic necklace with a tiny hourglass in its center is capable of transporting its user into the past, with each spin moving a wizard or witch further and further back. Through the use of such an object, one might reshape the world to fit their vision.
But there's a catch. In the Harry Potter universe, magic is governed by a series of quasi-Newtonian laws. When it comes to time travel, would-be Doc Browns are held back by Professor Croaker's Law, which states that a time-traveling magician is only able to travel back in time five hours at a time before incurring "serious harm to the traveller or to time itself" (via The Harry Potter Lexicon). In other words, you can go back to keep yourself from getting food poisoning, but not far enough to warn the corner store that they were selling expired Cheez Whiz last week.
Or you couldn't, anyway. This all changed with Harry Potter and the Cursed Child, and the introduction of a true Time-Turner.
Time travel is how you get more Voldemort
In Harry Potter and the Cursed Child, a play written by Jack Thorne and based on a story he devised with Rowling and John Tiffany, fans are transported forward in time to the far-off future of 2020, a sleepy little year when not much seems to be going on. In their fourth year at Hogwarts, Albus Potter and Scorpius Malfoy nab themselves the latest in magical accoutrements: a true Time-Turner. Created by Theodore Nott and commissioned by that greasy ne'er-do-well Lucius Malfoy, this new-and-improved model of Time-Turner has the capacity to move its user as far in time as they want, and allows them to stay there indefinitely. Nott is eventually arrested for creating something so powerful, which is a pretty damning statement in a world populated by dragons and trees that will punch you for no reason.
Needless to say, the introduction of consequence-free, unlimited time travel presents a whole new level of threat to the Wizarding World. The events of Cursed Child chronicle the shenanigans that a couple of Slytherins get into when they wrap their mitts around a true Time-Turner: Voldemort almost comes back, and his daughter Delphini gets into all sorts of bad business.
Essentially, this all-powerful tool serves to teach enchanters of all ages what sci-fi fans have known for years: Time travel is just too dangerous for anyone but the Doctor to meddle with.