A Space Marine Legion was one of the original, enormous formations of the Adeptus Astartes, the mighty armies with which the Emperor waged His campaign to reunite the galaxy. Where a modern Chapter numbers around a thousand warriors, a Legion at its height could field tens or even hundreds of thousands, entire hosts of superhuman soldiers.
There were twenty Legions, each eventually led by one of the primarchs, the Emperor's demigod sons, and each bearing the character and gene-legacy of its father. During the long crusade to conquer the stars, the Legions were the tip of humanity's spear, bringing world after world into compliance. But such concentrated power proved a terrible danger when half of the primarchs turned traitor, plunging the Space Marines into a civil war that nearly destroyed everything the Emperor had built.
In the aftermath, it was decreed that no such power should ever be gathered again. The surviving loyalist Legions were broken apart into the smaller Space Marine Chapters that endure to this day. The traitor Legions, meanwhile, fled into the Eye of Terror, where as the Chaos Space Marines they preserve their ancient structure and their hatred still. The age of Legions thus lives on only in the loyal orders' proud memory and in the enmity of the damned.