Zone Mortalis is the term used by the Legiones Astartes for the claustrophobic close-quarters warfare fought within the sealed corridors, bulkheads and boarding tunnels of void-craft, hive sub-levels and orbital installations. In such killing grounds there is no room for manoeuvre, no ranged supremacy, and no retreat, only the grinding attrition of blade, bolter and blast at point-blank range.
Fighting in a Zone Mortalis demanded specialised doctrine and equipment. Terminator-armoured killers, siege-breakers and terror weapons came into their own where corridors channelled the enemy into overlapping fields of fire and every sealed hatch became a fresh battle-line to be broken.
The boarding actions of the Isstvan campaigns and the void-war above Phall saw the Iron Warriors and their rivals perfect this brutal art, treating the Zone Mortalis as a discipline unto itself, where victory belonged to whoever could endure the abattoir of the tunnels longest.