Theta Cygni XII was the formerly inhabited twelfth planet in its star system.

Between 2067 and 2265, this planet was invaded by neural parasites that came from Levinius V. The civilization on Theta Cyngi XII was destroyed in the mass insanity caused by the parasites. The inhabitants transported this parasite over to Ingraham B. (TOS: "Operation -- Annihilate!")

This planet was only mentioned in dialogue.
According to Star Trek: Star Charts ("United Federation of Planets I") and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), the Theta Cygni system was a binary system in the Alpha Quadrant. The primary was a F-class star and the secondary was a M-class star. This was an independent system in the late 24th century.

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