Gilderoy heart dropped. Voldemort back? No? Surely not? It must be a mistake. Dudley must be confused. He couldn't be back. Surely.
Gilderoy stared into Dudley's eyes. His head was covered in blood, Gilderoy saw. There was no lie there. There was no signs of a confundus charm. He was telling the truth. He glanced down at Dobby. So, it was the elf who had rescued him?
He could use this, Lockhart realised. For the first time in his life, he felt a pang of guilt for what he was about to do. Dudley was his friend. But it had to be done. This was big.
"Obliviate," he said.
He cast the spell slowly. This called for precision. Immediately, Lockhart was going through the memories as if he reverse ... Lockhart's eyes gazing down ... appearing on the grounds ... then a gravestone shattering as a spell hit it—that accounted for the blood ... then calling for Dobby ... more memories ... Dudley running for his life ... a Death Eater falling ... Dudley punching a Death Eater ... Dudley using the fumos spell ... Lord Voldemort staring at Dudley, counting down—it was true. Lockhart had seen enough. He went back through the memories. The gravestone shattering—that was the moment. He would erase everything after that point ... no ... there was Dobby ...how to work this?
Lockhart went back through the memories, using his magic to erase the summoning of Dobby ... instead, he replaced it with Dudley sobbing behind the tombstone.
Dobby appearing with a sharp crack—erased. What to put there? Himself appearing in a swirl of color. Yes ... yes, Dudley would remember himself appearing. The spell shattering the tombstone—that would stay ...that was needed, to account for Dudley's memory loss—a blow to the head. Anything else?
Gilderoy went through the now modified memories.
...Dudley staring into Lockhart's eyes—that would stay.
...Dudley telling Lockhart that Voldemort was back—gone. Replaced with Dudley asking "Where ... where am I? What happened?" "Thank Merlin I found you!" Gilderoy Lockhart said. "Thank Merlin I got you out of there" Yes, that worked.
... Dudley appearing with Dobby. Gone ... wait, he needed Dobby. He added Lockhart apparating Dudley here with a tearful Dobby waiting for them. Yes ... it was Dobby who would have fetch him, that worked.
...the grave shattering—stay.
...a swirl of colorful cloaks arriving—hmm, maybe something else. "Stupedy!" cried Lockhart—yes, much more heroic if he drove off Voldemort first.
Dudley was done and Lockhart found himself drawing out of Dudley's memories. It had only taken less than a second to modify his friends memories.
Now for the elf. "Obliviate ..."
Lockhart did the same, going through the Elves memories and modifying them. How to make this work.
The oldest memory—Dobby hearing his master's voice calling for help, replaced. Dobby getting a sense that his master was in danger. Yes, that worked better.
Now, how did he tell Lockhart? Dobby apparating into the castle grounds - "Master Dursley is captured! You-Know-Who! Come quick, Professor!" "Lead the way, my friend!" Lockhart shouted, wand raised. "He'll wish he never tangled with me,"
Dobby waiting, sobbing while Lockhart apparated ... no, you can't apparate on Hogwarts grounds! Ok—Dobby apparating with him. Yes, that worked—he used Dobby to take him there and Dobby to bring him back.
Once more Lockhart went through Dobby's memories, making a few tiny modifications.
He disengaged.
"Professor ..." Dudley said, breathlessly. "You ... you came!"