The Chova was a powerful weapon created by Pad'gy D'ullh, a brutal dictatorial ruler of the planet Damiano.
The Chova was a telepathic weapon, with no physical presence. It was developed by genetically engineering a telepathic race called the Hed'em'disol, then inserting the Chova into their minds. The Hed'em'disol would then telepathically project the Chova to their victims. The weapon first prompted dreams, then waking hallucinations, based upon the victims' perceived personal failures.
When Pad'gy was overthrown, the Chova and almost all records of it were lost, and its existence became more legend than fact. In the late 24th century, however, political commentator Je'tran T'ullh was able to recreate the Chova, and used it to strike against the USS Enterprise. The Chova was inserted into the mind of Vulcan security officer Ensign T'Mor, who inadvertently spread it throughout the ship.
The Chova was defeated when Captain Jean-Luc Picard was able to use the multiple personalities that had been imprinted upon his mind -- Sarek, Kamin, and Locutus of Borg -- to overwhelm the weapon. (TNG comic: Perchance to Dream)