The Alpha Legion was the last Legion creatd during the First Founding, it being the 20th Legion. When Horus made his pact with Chaos, the marshall pride of the Alpha Legion was their downfall. The Warmaster was a mighty warrior himself, he commanded armies and fleets, and commanded at the forefront of the Emperor's wars. By comparison, he made the distant Emperor on Terra seem weak and cowardly. The Alpha Legion continues to fight a covert war against the Imperium to this day. They are connected with Chaos Cultist of many of the settled worlds of the Imperium. The Alpha Legion directs the activities of Cultists across entire sectors and they instigat massive insurrections against Imperial rule.
Codex Chaos 2nd ed.
The Alpha Legion was the twentieth and last legion created in the first founding. Under the critical eye of their Primarch during the Great Crusade the Legion became renowned for its discipline and strict organization. Though the youngest Legion, the Alpha Legion sought to outshine its brethren in all things as if to prove their worthiness amidst the older Legions. The Alpha warriors adopted the symbol of the hydra as their Legion's symbol. This many-headed, dragon-like creature from ancient myth served to remind the brethren of the Alpha Legion of their ultimate unity in body and spirit. On the battlefield the terrifying coordination of the Alpha Legion was their hallmark, their attacks kept the enemy under relentless pressure while they sought a weak point in their defenses. When Horus made his pace with Chaos the martial pride of the Alpha Legion was their downfall. The Warmaster was a mighty warrior himself; he commanded armies and fleets and fought at the forefront of the Emperor's wars. By comparison he made the distant Emperor on Terra seem a weak and cowardly individual. The Warmaster was a leader worthy of their respect, the Emperor sought only to exploit Horus's conquests and crush the liberated humans of the galaxy beneath his stifling regime. So the lies were insinuated into the hearts and minds of the Alpha Legion, and if any lie is repeated often enough it begins to be accepted, and once accepted it becomes truth. Joyously, the Alpha Legion clashed with loyalist Space Marines on Istvaan V and the campaigns thereafter. Here at last was an opponent fully as tough, as war trained, as ferocious as themselves. The brethren of the hydra inflicted stinging defeats on the loyalists at Tallarn, Yarant and dozens of smaller outposts before moving onward into the Ultima Segmentum like an all destroying comet. The Alpha Legion became entirely separated from the forces of Horus but continued to wage war on all that they came across. By the end of the Heresy they were inventing objectives and missions of their own to fulfill their war-lust without reference to their allies. Even after Heresy failed the Alpha Legion continued to fight a covert war against the Imperium. Small units of Alpha warriors set up hidden bases in asteroid fields, space hulks and barren systems throughout the galaxy while the bulk of the Legion withdrew to the Eye of Terror. Raiding parties sally out from these secret bases to catch the defenders of humanity unaware - sabotaging bases, attacking shipping, terrorizing settlements and destroying small outposts with deadly efficiency. Far more insidious and dangerous are their connections with Chaos Cultists on the settled worlds of the Imperium. The Alpha Legion coordinates and directs the activities of Cultists across entire sectors to instigate massive insurrections against Imperial rule. These revolts are often used as a cover for a series of shattering Chaos Space Marine raids or as a precursor to a full-scale invasion from the Eye of Terror. The Inquisition holds a special loathing for the Alpha Legion for their part in spreading these iniquitous demon cults and fanning the embers of heresy into the raging fires of outright rebellion.
