- "It appears that now even the Tau have gained a great evil to stalk them..."
- —Eldar Farseer
The Ka'tsu Sept is a Tau sept that was colonised by Tau forces during the third sphere expansion. Existing primarily as a halfway station between neighbouring D'yanoi and outpost sept Ke'tsai, Ka'tsu saw considerable military development with the continually rising tyranid threat approaching from the eastern fringe. By late M41, Ka'tsu was officially established as an independent sept with five fully-functional native castes.
Only 11 months after the Sept was founded with assistance from the Oresh'uan Sept, the world was plagued with a terrible mutating disease known as Rust. The source of the disease's origin is believed to be the Kroot regiments that had only recently been sent in. As of now, the world has been quarantined as the bulk of the population has entered the stage IV of the disease, and is being patrolled by several alternating Oresh'uan cadres. It poses a extremely high threat to the Tau Empire.
Unknown to the rest of the Tau Empire, a considerable resistance of healthy Tau still live on the planet, being directed by the young and valiant Shas'el Fo'taal, and are engaged in a ceaseless conflict to maintain their defensive, and indeed quarantined, position. Their only hope is that they will kill off the rusted population - before they themselves are infected.
- "Where did you say those blankets came from? Oh well, it's Sassy El Photo's problem now."