When the Space Wolves struck Prospero and destroyed the Lesser Light which protected the Thousand Sons, most of them died to the suddenly uncontrollable Warp they thought they had command of. Some few struck out for a hidden gate that would lead them to Terra and Magnus; many more went mad, mutated into something inhuman, or died.

It is a matter of opinion whether those who became known as The Thousand should be counted among the mad or as a separate category. What is fact is that they glimpsed the depths of the Warp and saw the flesh-change they had felt before Magnus met them was the blessing of Tzeentch, Changer of Ways and Master of Fortune, Lord of Magic and Secret-Shaper. While some undoubtably were driven quite mad by their glimpse of the natures of Change and Reality, others supplicated themselves before it and understood that this was what Magnus feared; embracing the greatness of Chaos and the nature of the universe, and its manifest superiority to the Emperor's vision of a 'perfect', static galaxy. The Great Conspirator smiled as its plots came to fruition in ways not even its instrument Vulkan suspected, and it directed its Lords of Change and new marine devotees to round up other marines who had not yet seen the light and convert them. These efforts proved very successful, and so, renouncing the name "Sons" and its ties to the blinkered, foolish Magnus, the Thousand were born. Already few in number before the Burning of Prospero, the Thousand actually numbered only 593 on entering Tzeentch's service. That being the case, they were largely uninvolved in the Heresy War, seen only in a few engagements alongside traitorous elements of the Hydra [Ed.Inq., PROHIBITORI: this was alongside the component led by the false primarch designated 'Betaos', believed to be the daemon known as The Changeling]. They only saw widespread deployment in the retreat from Terra, where they conducted large-scale rituals using the populations of planets in the path of the retreat to slow the loyalists and aid the traitors - particularly the Salamanders - in reaching safe harbor in the Eye of Terror.

In the centuries since, the Thousand have regained their numbers and probably number as much as twice their nominal complement. They do not fight as a unified legion, but instead serve as specialist units, most with the Salamanders but alongside all the other forces of Chaos except the Space Wolves and Death Guard. They have perfected a specialized style of sorcery by which a group can cast together to share the successes and mitigate the failures. This has also had the side effect of merging their consciousnesses to some extent, and many can speak telepathically even while in two places in the materium kilolightyears apart.