Because of the Dementors' unique nature, even Dumbledore found it difficult to eradicate them completely. Only Ancient Magic and Harry's magic could destroy them utterly.
If Harry had not been there, those Dementors would have been unable to harm Hermione. Of course, Hermione would not have been able to destroy them either; she could only drive them away.
But with Harry there, that would never happen.
Without drawing attention, Harry lightly moved his fingers, and a silver radiance unique to Ancient Magic merged into Hermione's Patronus Charm.
The Dementors trying to flee suddenly stiffened. Then they let out silent, piercing screams as their bodies rapidly turned to ash and scattered in the wind.
Hermione froze when she saw this, but quickly realized what had happened.
She ran to Harry's side, grabbed his hand, and asked, "Harry, you helped me destroy those Dementors just now, didn't you?"
Harry shook his head and affectionately reached out to stroke Hermione's little head. "No, that was the effect of your spell."
Hermione rolled her eyes at Harry. "I'm not going to believe that. How could I possibly be that powerful?"
Harry smiled but did not answer.
After the two had finished talking, Dudley cautiously approached and asked, "Harry, what were those things just now? They were terrifying."
Harry explained, "Dementors. They're very dangerous Magical Creatures. Unless it's necessary, try not to go out lately. There are quite a lot of them outside."
Dudley nodded thoughtfully.
After returning home, Harry and Hermione went back to their castle.
But not long after they arrived, a letter suddenly flew in from outside.
It was a Howler. A shrill female voice came from the envelope:
"Dear Miss Hermione, the Ministry of Magic has learned that at six twenty-three this evening, you cast the Patronus Charm in front