The Thunder Warriors
The Thunder Warriors, also sometimes referred to in ancient records as the Thunder Legion, were the first gene-enhanced warriors created by the Emperor and served as the precursors to the present-day Space Marines.
The Emperor wrought these proto-Astartes to be faster, stronger and more powerful than any of the feral barbarian gene-sept warriors that claimed fealty over the various techno-barbarian nation-states of Old Earth. These warriors were a gestalt mix of unprecedented superhuman physical power, gene-programmed resistance to environmental and even psychic attack, a warlike spirit and the Emperor's own strategic genius.
The Thunder Warriors were known to be even stronger, more savage and more potent in combat than the later Astartes, though they were not as long-lived and suffered from often dangerous mental instability and early metabolic collapse when their bodies began to reject their augmentations. They were also created from adult male supporters of the Emperor rather than the adolescents used to create the Astartes.
Imperial chronicles hold that the last of the legendary Thunder Warriors died heroically during the final battle of the Unification Wars in the middle years of the 30th Millennium that reunified Terra beneath the single rule of the Emperor and forged the government that would become the Imperium of Man. Unfortunately, this heroic version of events was completely false. The Thunder Warriors had not died to a man during the final battle of the Unification Wars. They had been brutally culled from existence by the Emperor's own servants on His orders. The Thunder Warriors had been destroyed by their own creator, a secret skillfully concealed from the people of the Imperium for more than 10,000 Terran years.