Warhammer 40,000: First Strike (2017), p7 — War Zone Ultramar

Ultramar is a sub-sector of the Imperium ruled by the Ultramarines Chapter of the Adeptus Astartes. It is among the most heavily fortified and defended regions of the Imperium, yet it is in the throes of a pestilential invasion by the Death Guard.

During the Creeping Doom offensive, the plague-ridden Chaos Space Marines wrought untold destruction on Ultramar. Daemonic poxes ravaged planets and virus bombardments turned hive worlds into rubble-strewn cesspools of disease.

Nurgle, the Chaos God to whom the Death Guard are aligned, sees in the fertile planets of Ultramar the perfect breeding grounds for his most vile contagions. It falls to the Ultramarines to defend their realm, and to prevent agri worlds such as Masali from becoming flyblown wastes.