The Jerulas Crusade was led by High Marshal Ludoldus in 645.M39. The Crusade saw the development of the Land Raider Crusader. Jerulas was a long-isolated system, rediscovered by the Imperium. Attempts were made to integrate the worlds into the Imperium, but the system had long been prosperous and independent, and scorned Imperial interference. The missionaries sent to bring the system into the greater fold of mankind were killed. More missionaries came, this time their words backed up by the force of the Black Templars.
The crusade subdued all the surrounding worlds and closed in on Jerulas itself. The planet was a well-fortified hive world and each besiegement undertaken by the Black Templars caused them heavy losses.
The crusade became a long siege, a situation the Templars were not suited for (unlike their founding chapter, the Imperial Fists). The defenders' claim that their fortifications were designed by Rogal Dorn himself during the Great Crusade made the Black Templars more determined to win the battle. Eventually, ancient techno-arcana was discovered, allowing the Marine Artificer Simagus to create the Land Raider Crusader. This new type of Land Raider enabled the crusade to smash through the defences of the hive cities. Eventually all the hive cites fell or surrendered.