Nimbosa was a large Imperial Civilised World roughly the size of the Sol System's gas giant of Saturn in the Ultima Segmentum of the galaxy that was overrun by the T'au in the late 41st Millennium. It was retaken by the Imperium during the successful Nimbosa Crusade that was led by elements from several Chapters of the Adeptus Astartes, including the Black Templars, Imperial Fists and the Ultramarines.
There is little information in regards to the early history of Nimbosa, other than that it was a vitally important Civilised World located in the Ultima Segmentum near the borders of the T'au Empire. When Hive Fleet Kraken began to threaten worlds on the Eastern Fringe of the Imperium of Man in 992.M41, the High Lords of Terra responded by stripping the worlds on the border with the Tau Empire of Imperial forces to respond to this xenos threat.
The T'au Empire was quick to recognise this redeployment and soon moved to take advantage, beginning the period of T'au history known as the Third Sphere Expansion. The T'au had soon annexed a whole swathe of Imperial worlds along the border. With the majority of Astra Militarum and Imperial Navy units redeployed to fight the Tyranid threat, the T'au landed a vast invasion force on Nimbosa, Commander Brightsword, the leader of the T'au, set about the systematic extermination of the populace.
The only Imperial Guard force able to resist them was the Vostroyan Firstborn's IXth Regiment, who made their first stand in the factory city of Polia. Serving under General Graf Toschenko, the Firstborn knew that against such an overwhelmingly large invasion force their only hope was to hold out against the T'au long enough for reinforcements to arrive: something that could take solar weeks if not months to happen.
The fighting culminated with the infamous Koloth Gorge Massacre, where the forces of the Imperial Guard were so brutally slaughtered that Brightsword was censured, summoned back to T'au due to the remorselessness of his acts. The Imperium launched the Nimbosa Crusade in retaliation, an Imperial Crusade undertaken in 993.M41 by the Black Templars, Imperial Fists, Raven Guard, and Ultramarines Space Marine Chapters against the T'au Empire following the T'au conquest of Nimbosa. All of these Chapters have reputedly fought the T'au on the heavily contested planet at one time or another and the world has changed hands many times and still remains a war zone.
- Codex: Imperial Guard (5th Edition), pg. 22
- Codex: Space Marines (6th Edition), pg. 45
- Codex: Tau Empire (7th Edition) (Digital Edition), pp. 67, 80
- Codex: Tau Empire (4th Edition), pp. 14-15
- Warhammer 40,000 - Cities of Death, pp. 56-57
- White Dwarf 314 (UK), "Marshals of the Great Crusade - Folker," pg. 96
- White Dwarf 312 (UK), "Righteous Victories," by Graham McNeill, pg. 19
- Assault on Nimbosa (Short Story)