During the Galactic Civil War in Star Wars, there were heroes and villains as powerful as Darth Vader. While with the Empire, he was left for dead, betrayed and sent on many suicide missions. But against all odds, Vader survived as a statement that he couldn't be beaten. But one theory believes that even with all of his strength, there was one thing he couldn't have expected that led to his most significant defeat.
Star Wars: Episode VI - Return of the Jedi features the final battle between Darth Vader and Luke Skywalker, who has now become a Jedi. During the battle, Vader's strength is on full display until Luke's rage takes over. But according to Reddit theorist REdd06, Luke's rage wasn't Vader's downfall. Instead, it was the realization that Leia was his daughter.
Throughout the original trilogy, Leia's status as Princess and General of the Rebellion put her in the Dark Lord's sights on multiple occasions. For example, during Star Wars: Episode IV - A New Hope, Vader captures and tortures Leia to find the Death Star plans. But when she refuses, he destroys her home planet, Alderaan, as a show of Force. Of course, at the time, he had no idea that he was torturing his daughter. But if he did know, would he have hesitated?
Vader showed no remorse early in the war because everything he loved as Anakin had died. As a result of thinking he had nothing worth living for, he leaned heavily into the Dark Side. But when Luke appeared, seeds of doubt began to sow in the Sith Lord's mind. After his battle with Luke in The Empire Strikes Back, the Dark Lord struggles with bringing Luke to his side or defying the Emperor and going his own way. By the time he fights Luke for the last time on the second Death Star, he's handicapped by his wavering faith in the Dark Side.
When Vader searches Luke's mind and learns that Leia is his child, the theory believes this cut Vader to his core. Outwardly, he coaxes Luke into fighting by saying he will convert her instead, but deep down, it's possible that is all a ruse. Knowing the atrocities he committed against his daughter could have been enough to guilt him into staying with the Dark Side, where he believes he deserves to be. If true, it's shown by how he carelessly defends Luke's onslaught, welcoming defeat. But Luke's persistence that he's still good and can be redeemed helps him realize he can still do one thing for the right reasons.
While Leia never forgives her father for his actions, it's hard to ignore that his final sacrifice was for them. Saving Anakin was a family effort, and while Leia wasn't physically with Luke, her presence could have been enough to help him. There's no confirmation that the theory is correct but seeing the emotion of the duel with a lens that Leia was the reason Vader is saved adds powerful context to the Skywalker Saga and serves to enhance the moment.