Spirit Ball

Spirit Balls are malediction ghosts that appear in Luigi's Mansion: Dark Moon. During his adventure to piece together the Dark Moon, Luigi comes across some places that have invisible objects. In order to reveal them, Luigi must use his Dark-Light Device. When the light hits the invisible object, Spirit Balls come out. Luigi must use his Poltergust 5000 to suck them up in order for the invisible object to fully revealed. If he doesn't suck the Spirit Balls in a certain amount of time, they go back to the object and turn it invisible again. Spirit Balls occasionally hurt Luigi if he gets too close, dealing 5 HP of damage. In a mission in Gloomy Manor, it's seen that a Boo can produce some Spirit Balls, implying that Boos can make illusions just like King Boo does. Unlike other ghosts, they cannot be sent to the Vault, nor do they appear in the ScareScraper.

Trivia

 * Spirit Balls may be based off of the Will-o'-the-wisp.