The famous Grandmother Paradox goes like this:
Imagine you travel back in time, to a point before your father was born, and kill your grandparents. This action creates a paradox: by killing your young grandmother, you prevent your father from being born, which also means you will never exist to kill your grandmother in the first place.
Or, in other words: if your existence implies your grandmother survived, then how could you have killed her?
Tywin's use of the Time-Turner almost led to a similar paradox. Unknowingly, he ended up injuring himself in his own attack.
The version of Tywin who traveled back hadn't grasped the complexity of the situation.
All he saw was another "himself" approaching, and he assumed that must be the assailant, so he raised his wand to strike first.
But in fact, that was his original self.
If Tywin had survived, he might have looked up to the sky and cried, "Who killed me? And whom did I kill?"
Seated in Dumbledore's office, William continued analyzing the entire series of events.
"After attacking 'himself,' Tywin faced no more obstacles and proceeded to the entrance of the Ravenclaw common room, where he killed Robert and took the door knocker.
"By this point, he had already battled 'himself' three times and was badly injured. He drank the Polyjuice Potion he'd prepared in advance, transforming into Professor Snape, and headed to the storeroom."
"Why did he disguise himself as Snape?" asked one of the former headmasters from a portrait.
William recognized him as Armando Dippet, the previous headmaster.
"It's simple. Tywin had already fought 'Snape' in the fifth-floor corridor and didn't realize it was actually himself using Polyjuice Potion. But as far as he knew, he had fought Snape.
"Since a confrontation had taken place, Tywin assumed Snape would go to Dumbledore's office afterward, leaving the storeroom unguarded.
"So, he took on Snape's appearance, hoping to grab some healing potions and use the disguise to escape with the door knocker."
But to his surprise, the real Professor Snape was still in the storeroom, and they ended up dueling.
This was the first memory Dumbledore had shown William.
Snape had recounted how someone who looked exactly like him had tried to break into the storeroom, and the two had fought.
Tywin, severely wounded, hadn't been able to hold his own against Snape and had taken a hit to the chest from a Dark curse before escaping. Snape then headed to Ravenclaw Tower and found Robert's body.
"That was when Tywin used the Time-Turner a third time, intending to stop his original self from taking the ring. But, as fate would have it, he was mistaken for Snape by his past self, who had just left the Defense Against the Dark Arts office."
William stirred the Pensieve with his wand, conjuring the memory of Tywin and "Snape" fighting in the fifth-floor corridor.
"That was Tywin's fifth injury," he said. "Realizing he was caught in a time loop, he tried to hide in the Room of Requirement—only to find Fluffy waiting inside."
"Professor, that's how everything unfolded. Did I miss anything?" William asked, looking up.
Dumbledore began clapping, and the other headmasters in their portraits joined in, except for Phineas Nigellus Black, who snorted and called Tywin an idiot.
"William, you're even more perceptive than I thought," Dumbledore said approvingly. "I suspected you might need more time to put the pieces together. Truly, you are a Ravenclaw through and through."
"Oh, it took me long enough to figure it out," William said modestly. "You would have seen the truth at a glance."
"You're giving me too much credit," Dumbledore replied with a warm smile. "I am hardly infallible; many possess wisdom I do not.
"Although," he added with a chuckle, "I don't think I've blushed this much since Professor McGonagall called me noble."
William took a breath and continued. "Professor, I have one question."
"Yes?"
"If Tywin had a Time-Turner, why would he risk everything for the supposedly mythical Ravenclaw ring? Couldn't he just use the Time-Turner to travel back in time?"
Dumbledore shook his head. "Do you truly believe that?"
"Isn't that how it works?"
"I imagine you've come across one particular phrase repeatedly in your research on Time-Turners?"
"'Prohibited from misuse.'"
"Exactly," Dumbledore said, folding his hands and leaning back in his chair.
"The Ministry's studies have shown that the longest permissible time one can travel back without causing serious harm to the traveler or to time itself is about five hours. Moreover, each period should be traveled through no more than twice.
"Attempts to travel back further have led to catastrophic consequences for the involved witches and wizards.
"These experiments were halted in 1899 when Eloise Mintumble found herself trapped in the year 1402 for five days. Her body aged five centuries during her return trip, suffering irreparable damage. She died soon after being retrieved, despite the best efforts of St. Mungo's.
"During her five days in the past, she severely disrupted the lives of everyone she encountered, impacting their descendants in such a way that at least twenty-five individuals were erased from existence."
"After she returned, disturbing effects continued for days. Time itself had been destabilized—Tuesday stretched for two days, while Thursday shrank to only four hours.
"The Ministry had quite the ordeal covering it up, and as a result, imposed stringent laws around time travel research and use."
"Even within the Ministry's strict regulations, Time-Turners come with hundreds of restrictions."
"Although traveling back five hours or less is safer than a five-century jump, it can still have dangerous consequences.
"As Saul Croaker once said, 'The human mind can't comprehend time, nor the havoc it can unleash when tampered with.'"
William nodded as Dumbledore spoke.
Tywin hadn't even traveled five hours—only forty minutes each time, using the Time-Turner just three times, and yet it nearly resulted in a paradox where he inadvertently attacked himself.
If he'd tried to go back ten years, the potential for unforeseeable chaos would have been immense, which explained why he was so desperate to find the Ravenclaw ring.
After all, according to legend, the Ravenclaw ring allowed time travel without any of the usual side effects.
A sudden realization struck William: the whole reason Tywin had resorted to using the Time-Turner in the first place was Dumbledore's unexpected visit to the Ravenclaw common room.
This single decision had set off a cascade of disastrous events for Tywin.
Thinking it over, William felt a chill.
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