The Children's Crusade, also known as the Lost Crusade, was an unofficial Crusade in the Imperium in 754.M37. On a thousand planets, menials rebelled against their Administratum masters. The insurgents demanded a better way of life and a galaxy of tolerance. The movement was especially popular among youth, earning the title of the Children's Crusade. Billions of earnest pilgrims were lured to seek transport to Terra; some by pirates. However, the majority of their transports disappeared in the Warp in transit.
Later, in 666.M38, the armada suddenly reemerged from Warpspace near Terra. Twisted and mutated, many of the hulls had fused together. What fate exactly befell the pilgrims was unclear, as the Inquisition worked feverishly to cover up the incident and purge the flotilla's cargo of abominations.