* The ghosts of Cedric Diggory and his parents: This was a direct result of Voldemort's resurrection ritual, where Cedric's death triggered the ritual and resulted in the ghostly return of Voldemort and the other victims of the event.
* The ghost of the Headless Hunt: This was more of a side effect of Voldemort's return. The ghosts of the Headless Hunt were drawn to Voldemort's power and appeared alongside him, as they are a force of darkness.
The reason Harry was able to see these ghosts was because of his connection to Voldemort:
* Shared connection to death: As a Horcrux, Harry was deeply connected to Voldemort and his power. This connection made him more sensitive to the world of death and ghosts, allowing him to perceive them.
* Voldemort's presence: The presence of Voldemort, who was a powerful dark wizard, amplified the ghostly presence and made it more visible to Harry.
It's worth noting that seeing ghosts is not a common occurrence in the wizarding world. Most witches and wizards cannot see ghosts, and those who can are often considered eccentric or strange. Harry's ability to see ghosts was directly tied to his unique relationship with Voldemort and his involvement in dark magic.