I want a Harry Potter fic where Petunia and Vernon are killed in Harry’s 6th year, and Dudley is injured and brought to the Hospital Wing at Hogwarts to heal. And Dumbledore says he’s going to find somewhere for Dudley, but Dudley wants to stay at Hogwarts so he can be with the only family member he has left. And Dudley just hangs out at Hogwarts, sometimes going to classes with Harry, and sleeping in the boy’s dorm in Gryffindor. Harry even lets Dudley ‘practice spells’ with his wand and they laugh when Dudley gets disappointed when nothing happens.
But then Harry starts having his ‘private lessons’ with Dumbledore, and one night, Harry wakes up Dudley, and says that he’s going to meet Dumbledore, and doesn’t know when he’ll be back, so if he isn’t back by class-time, can Dudley go to his classes and take notes. And Dudley agrees.
Only, hours later, Ron wakes him up, saying there’s Death Eaters in the castle, Dudley, you need to stay in the dorm. And everyone disappears to go fight the Death Eaters. But Dudley is concerned about Harry, so he tries to find him. He sees some Death Eater knocked out and takes his wand, just so that he can’t have it when he wakes up. Only one of them goes after him, and he throws out his hands on instinct, and what do you know. The wand works! So he uses some of the spells he’s learned.
And someone starts screaming that Dumbledore is dead, and oh god Harry! So he goes outside where everyone is around Dumbledore’s body, and Harry walks up, and Dudley can’t do that light bit that everyone is doing, but he can comfort Harry.
And later they find out that Petunia had made a deal with Dumbledore after she discovered Dudley doing accidental magic at a young age. She said she would continue taking care of Harry if Dumbledore suppressed Dudley’s magic. So the very moment Dumbledore died, Dudley’s magic was no longer suppressed.