The representation of the Chova in Picard's psyche.
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. The only known cure for the Chova was for it to infect someone who suffered from multiple personality disorder. Since the Chova could only attack one personality at a time, multiple personas would overwhelm the Chova, essentially "negating" it; the negated Chova would subsequently be passed on to other infected to cure them.
When Pad'gy was overthrown, the Chova and almost all records of it were lost, and its existence became more legend than fact. However, in 2371 political commentator Je'tran T'ullh was able to recreate the Chova, and used it to strike against the USS Enterprise-D as they were interfering with his attempt to assassinate the planet's new leader. 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 Doctor Beverly Crusher learned of the Chova's weakness to MPDs, and asked Captain Jean-Luc Picard to allow himself to be infected, reasoning that his past experience with a traumatic mind meld, an extensive memory graft, and his assimilation by the Borg—resulting in elements of Sarek, Kamin, and Locutus of Borg remaining within his subconscious—had given him the makings of an MPD. Despite the risk in allowing Locutus in particular any chance to get out of Picard's mind, Picard agreed to the procedure, the four personalities overwhelming the Chova with relatively little effort. (TNG - Perchance to Dream comics: "By a Sleep to Say We End", "In the Sleep of Death, What Dreams May Come", "Enterprises of Great Pitch and Moment")