The "Critters" starship was an interstellar vessel or a living entity which originated extremely far away from the galaxy or possibly from another dimension. It was the source of the "Critters" and threatened all races in the Milky Way Galaxy during the 23rd century.
One such vessel or extra-dimensional being was known to exist. It based its operations in a region near the Romulan Neutral Zone which contained Watchtower 13. The vessel moved through space by generating wormholes and kept a doorway open to its point of origin.
It had the shape of an oblate spheroid measuring roughly 10,000 by 8,500 meters. It could extend tentacles outward at least 5,000 meters to cocoon and dismantle enemy vessels. It appeared to be the source of projectiles used to pierce enemy hulls. The tips of the projectiles then opened so that several Critters could disembark.
Deep within the vessel's interior was a command nexus, navigational equipment, and an area where prisoners were cocooned and genetically altered. The interior appeared mostly organic but also contained some integrated mechanical components.
In 2257, following the loss of the Constitution-class USS Farragut to the Critters, Starfleet ceased building Watchtower bases and avoided the sector. Critters subsequently attacked ships from the Klingon Empire, Romulan Star Empire, Gorn Hegemony and First Federation. Insufficient sensor data and clandestine attacks spread out over many decades helped disguise that one race or entity was responsible.In 2286, Kor’s IKS Revenge and T'Cel’s ChR Phoenix united with James T. Kirk’s USS Enterprise-A in the Watchtower sector to confront the Critters. Revenge posed as bait, and shortly afterward it was attacked by Critters. Then the Critter vessel appeared and snared the Klingon warship with its pseudopods.
Kirk, Spock, Saavik, Leonard McCoy and T'Cel beamed onto the command nexus of the alien vessel to investigate. Kirk perceived it as completely alien. No crew were detected aboard the vessel, though Saavik pointed out that the crew might be beyond their ability to perceive. Kirk wondered if the vessel was a single, living entity, similar to an organic version of V'Ger. Spock located navigational charts, but couldn’t match the data with known celestial or galactic references. He postulated that the vessel’s point of origin was so distant that the Milky Way Galaxy was not apparent on their charts.
The Critter vessel opened a wormhole to return to its origin point. With T'Cel aboard, Phoenix quickly flew ahead through the rift to investigate the other side. When the Critter vessel entered the event horizon of the rift, Kirk fired a doomsday weapon at Revenge. The annihilation of Revenge's warp engines combined with the weapon’s blast to trigger a supernova-like explosion on both sides of the wormhole. The blast destroyed the Critter vessel and permanently closed the wormhole. (TOS comic: "Debt of Honor")
In 2308, T'Cel was aboard Phoenix when it orbited Qo'noS to rescue imprisoned Jamie Finney. It was not known how they returned to the Beta Quadrant, or whether they had located other Critter vessels. (TNG comic: "Cry Vengeance")