Cambolinee
Colonized 2086 CE by a private group, the Cambolinee Foundation. The location of the planet was never reported to authorities, and was a mystery for many years.
In pre-Imperial days, when Terra was still considered nominal leader of the loose hegemony of human colonies, and when Cambolinee still trafficked with other human worlds (although its location was even then a well-kept secret), a delegation from BDA Tr#ska was attending a formal ball on Ibidem when delegates from Cambolinee, Terexta, and Promethia ridiculed the Tr#skans' skin and eye color. The population of BDA Tr#ska swore vendetta against the three worlds. After Terexta was attacked and all but destroyed (later to be rebuilt by the Empire), Promethia opted to pay reparations to the government of BDA Tr#ska. Promethia finished paying these reparations in 2461 CE, for a total of over 900 trillion Imperial Dollars. Cambolinee, apparently relying on its secret location as an ultimate defense, offered nothing to BDA Tr#ska, and from that point on, the people of BDA Tr#ska were determined to make Cambolinee suffer.
In 2153 CE, Cambolinee declined to join the First Terran Empire. By TE 120, the Empire controlled every Human-colonized planet in the Galaxy except Cambolinee. The Imperial Navy spent decades searching for the planet, with no success.
Finally, in TE 168, a covert Imperial operation released spores of Kastechian Plague into Cambolinee's atmosphere, wiping out the settlements and rendering the planet contaminated forever after.
Some Cambolineeans were rumored to have escaped, but no Imperial confirmation of this was ever made.