The Poltergeist King appeared at an award ceremony to present Simon Belmont with his family's heirloom weapons. However, he instead denounced and cursed the Belmont family, claiming that he defeated the Count and they stole the credit for his accomplishment. He then disappeared, taking the weapons with him.
Soon after, it was revealed that the Count had kidnapped the real Poltergeist King, stolen his magic staff, and magically disguised himself at the award ceremony. He used the disguise a second time to threaten Kevin, Simon, and the Old Wizard as they searched for the real Poltergeist King, but Kevin dispelled his magic by blasting a stalactite and drenching him with water.
The real Poltergeist King was eventually rescued from captivity and appeared at the second award ceremony, where Simon's drawn-out acceptance speech bored him to sleep.
The Poltergeist King did not actually appear in Castlevania III: Dracula's Curse. He is mentioned only in the English translation of the game's instruction manual, which states that he is responsible for hiding the Belmont family's weapons throughout Castlevania. The Japanese manual never states such a character as existing, with the Pope sending Simon's ancestor on his mission.