The Alpha Legion was the 20th and last of the original founding Space Marines. The Alpha Legion was assigned a home world, whose name was never given, and was never colonized. During the Great Crusade they were based on their own fleet. Their armour is a regal blue with silver trim. Their Primarch was Alpharius.
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 organisation. 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 co-ordination of the Alpha Legion was their hallmark, their attacks kept the enemy under relentless pressure while they sought a weak point in their defences.
When Horus made his pact 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 Emperors 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 onwards 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 fulfil their war-lust without reference to their allies.
Even after the 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 scattered throughout the galaxy while (he 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, terrorising 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 co-ordinates 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 daemon cults and fanning the embers of heresy into the raging fires of outright rebellion.
Taken from Codex: Chaos v2
The Alpha Legion was the twentieth and last legion created in the First Founding. Though the youngest, or perhaps because of it, the Alpha Legion sought to outshine its bretheren in all things, as if to prove their worthiness amidst the older legions. Their pride led to a number of incidents where members of the Alpha Legion clashed with and even fought against members of other legions in existence at the time. In particular there seems to have been a strong mutual hatred between the Alpha Legion and the Imperial Fists as they had clashed on more than one occasion prior to the Heresy.
When Horus made his pact with Chaos, the martial pride of the Alpha Legion was their downfall and they were among the first to join him. The legion clashed with loyal Space Marines on Istvaan V and in the campaigns thereafter, any need for restraint forgotten. Even after the Heresy failed the legion has continued to fight a covert war against the Space Marines and there are dozens of well-documented instances of their raiding parties sallying out from secret bases to catch the defenders of Humanity unaware: sabotaging bases, attacking shipping fleets and destroying small outposts with deadly efficiency. Taken from Codex: Chaos Space Marines v3