The Rusalka Incident

Near the end of the 41st Millennium, the eastern fringe planet of Rusalka was beset on all sides by traitors, and the guardsman garrisoned there were rescued by a Tau force.

Rusalka
Rusalka is an ice planet, near the rim of the Eastern Fringe in the Northesia system. It's climate is cold all year round, with a few weeks of a slightly warmer period and a few days a year wherein it is so cold that if the thin amount of oxygen becomes a liquid. A day on Rusalka is 36 hours and 27 minutes precisely. It is the fifth planet in a system of 12.There are many mountains on Rusalka and only a little bit of unfrozen ocean. The Northesia system is fairly close to Ultramar, and so the Imperium decided to build a large outpost there, and since Rusalka is the only habitable planet in the system, it was the one to be used. An imperial guard regiment was founded and tasked with defending it, the 1st Rusalkan regiment. They have many orbital planetary defence weapons, as their purpose is to defend Ultramar at all costs. Some native animals are:

7 foot long mountain bears with exoskeletons as hard as steel,

gargantuan, whale-like, white spotted sea monsters with a combination of teeth that filter out plankton and sharp canines. Internal body temperature freezes a layer of water around their bodies, making a foot thick layer of frozen armour,

numerous game,

countless species of small fish and plankton.

The Rusalka Betrayal
For years after the founding, there were many secretive chaos cults on Rusalka. The commanders were aware of this, and requested the honour of the presence of the Ordo Hereticus. Inquisitor Thistlius Cattlius came to rid Rusalka of Heretics, however the newly promoted Governor Militant of the regiment was corruped by Slaanesh, and he assassinated the Inquisitor and impersonated him. Rusalka was under full scale war from itself, and situations were getting worse, with the loyalists running out of supplies.

The Fall of Rusalka / The Glorious Rescuing
A force of Tau, including 3 battalions of Fire Caste, a batallion of allied Demiurg, a large amount of Earth Caste builders and only 1 Ethereal were cut off from the Tau Sept of Har'Makah, lost and wandering the Eastern Fringe in search of their allies. They came across Rusalka, and struck the planet by surprise. The chaos forces were in shock, and quickly fell under the plasteel boots of the Tau. the Tau offered the Guardsmen a place among the Tau Empire, and the new (not heretical) Governor Militant accepted the offer, scared and amazed by the firepower of the Tau. The Tau accepted that they would never see their old Sept again, and painted their armour with the black and red of Rusalka's troops. The Imperium knows this incident as 'The Fall of Rusalka', but the Rusalka Alliance knows it as The Glorious Rescuing.