I don't own Harry Potter or Lord of the Rings. Though I would love to, JRR Tolkien and JK Rowling still own their awesome book series.
This is only sort of a crossover. You will probably enjoy it more if you've read LotR, but you don't need to.
Albus Dumbledore swore he was going to find which idiotic muggle had written this Lord of the Rings. It seemed to be a new fad with with all the muggleborns. They had all walked in like they usually did, staring around at the entire room. And then they would look at him and gasp. "Gandalf!" He knew it was from some book, he had found that much out.
It started before the sorting one year. Albus Dumbledore was waiting in the entrance hall like he always had, waiting to lead all the first years into the hall. The first few came in, the purebloods and halfbloods, keeping their cool, making an effort to not look amazed. Still just like normal. And then came the muggleborns, staring around in wonder. Then one of them looked at him. She started whispering to her friend. "Gandalf!" That little friend gasped soon after. All the other muggleborns then turned towards him, amazed. Their little mouths dropped open too. "Oh my god!" one screamed, "It's Gandalf." She gave a little scream of happiness and fainted into her friends' arms. They too gave little screams. The wizarding kids gave them dirty looks and moved further away.
All while this was happening, the great hall was waiting. Behind Dumbledore, the doors opened. Slughorn opened the doors and asked, "Are the first years coming for the sorting?" "Yes, yes" Dumbledore said with a little shake. Slughorn gave a nod and waddled back into the Great Hall. With one more look at the fainter,
Dumbledore led the first years into the hall. The next day, when the fainting girl walked into Dumbledore's class, she started bouncing a little at the door.
When Dumbledore asked her to go to her seat, she jumped a little more, ran up and hugged him, and then went her seat. Her friends then proceeded to do the same. Dumbledore just stood there very confused. The same thing happened the next first year class, with a different group. And the next class. And the next.
That night, he found one outside his quarters. How had she found them? It was the first night of the year. He walked out and asked her, "Who did you think I was?" That he needed to know. "Undercover Gandalf!" she squealed, and tried to run off. He grabbed her shoulder. "And why..." But he didn't get to finish. The girl jumped up, gave another one of those little screams he was becoming slightly scared of and ran off. He stood there in a little daze. What just happened? And who was Gandalf? "Undercover Gandalf," He mused to himself. He walked back into his rooms and went to bed. The next morning, in his first class, he stayed in his office to avoid the squealing girls. When he walked in he was greeted again by squeals of, "Gandalf!" He shook his head slightly."Could any of you tell me who this 'Gandalf' you think I am is?" Again with the squealing, "Only the best, awesomest wizard from the best, awesomest book ever!"
Dumbledore winced at the sound. Now he at least knew who they thought he was. "I assure you, I am not Gandalf." The girls just smiled indulgently. The next day was even worse. Everywhere he went, he was followed by a crowd of first year muggleborns. Where had they come from? He didn't remember seeing them at the sorting. His nerves were being stretched.
That night, while doing his rounds, he was ambushed by a crowd. Only the scream of, "Gandalf" warned him of the impending danger. This was it. He ran towards the nearest window, jumped out and used a variation of the levitation spell to fly to the astronomy tower for a moment of peace. But even there, his new stalkers were waiting. Where had they all come from? Now it was his turn to scream. He turned his spell and flew off into the night. That was the fastest the levitation spell had ever been seen to carry someone. No one saw him for a few years. When he came back, everyone had been warned to never mention the name 'Gandalf' within any distance of him.
Lucky Grindlewald never read any muggle books, otherwise he would've had a very easy way of defeating Dumbledore when they first fought. Albus was never really the same.