Su Er studied the Tom Riddle who had appeared before him with curiosity, without saying a word.
Unlike his present-day appearance, without a nose or hair, Riddle was still in his prime. Though his hair was visibly thinning, his nose was still there. His eyes, however, had already grown narrow and elongated. Even in his translucent, colorless soul form, a faint red glow could still be seen covering those eyes, whose pupils had already become snakelike.
Just as Su Er was observing the Riddle before him, who was also Voldemort,
Riddle opened his mouth. Unlike a ghost or a living wizard, Riddle existed in a state between life and death, and his voice drifted as though it had no substance, yet reached their ears with extraordinary clarity.
"Long time no see, Headmaster Dumbledore."
He had not even called him Professor.
According to something Su Er had read on a forum, personally confirmed by Rowling Who Doesn't Understand Magic, Riddle seemed to have made this crown into a Horcrux after graduating, having obtained it in the Castle by winning the trust of the Grey Lady, Helena Ravenclaw.
[Setting change: originally, Helena Ravenclaw hid the Diadem in Albania. Here, I changed it so that the Bloody Baron accepted Rowena Ravenclaw's task of bringing back both her daughter and the Diadem. But you all know how that ended. The Bloody Baron was too rough and accidentally offed Ravenclaw's daughter.]
Riddle had planned to stay and teach after graduating, but Dumbledore turned him down, so he disappeared with the Diadem. A few years later, he returned to apply again and was rejected once more. It was also during this period of absence that the Diadem, already made into a Horcrux, was left in the Castle.
As for why Riddle had not placed defensive magic