The Poltergeist King guards the Belmont family's sacred weapons until they are needed again. He leaves them throughout the castle behind pillars. This character is only referenced in the English instruction booklet for Castlevania III: Dracula's Curse and was never actually mentioned in the original Japanese instruction booklet, thus his inclusion is considered non-canonical.
Captain N: The Game Master
The Poltergeist King has a prominent role in Episode 29, "Return to Castlevania", of Captain N: The Game Master. In it, Simon Belmont is about to be bestowed an honor in front of a large crowd for being a worthy hero like his great-grandfather Trevor by the Poltergeist King; however, The Count kidnaps him previous to said event and takes his form, accusing his ancestor of taking false credit for his slaying of him (the Count) many years ago.
In one scene, the Poltergeist King (or rather, the Count in disguise), appears as a whirlwind. This suggests that the Poltergeist King may have been responsible for the tornado that helped Simon Belmont to get around the Deborah Cliff in Castlevania II: Simon's Quest.