Alpha Legion
From Warhammer 40k
The Alpha Legion are the Chaos Space Marines legion about whom the least is known. They are experts in infiltration and their armies contain many Chaos cultists in addition to regular chaos space marines.
The Alpha Legion's primarch was named Alpharius, a brilliant and secretive Primarch who was reportedly later killed by Roboute Guilliman, the Primarch of the Ultramarines.
[edit] History
Alpharius and Omegon were the final primarchs to be found, just prior to the end of the Great Crusade and the beginning of the Heresy. Horus, Primarch of the Sons of Horus as his legion was then known, followed an escaping group of spacecraft into an asteroid field where they were set upon by multiple smaller craft. Despite being overpowered and outnumbered, Horus's opponents managed to flank him at almost every turn and eventually his own command craft was boarded. A group of five men, the fifth much larger than the rest, burst onto Horus's bridge, and Horus promptly slew four of them. The fifth managed to dodge Horus's shot and the two nearly came to blows, when Horus recognized a fellow primarch.
In the Horus Heresy Novel "Legion", it is mentioned that Apharius and Omegon share the same look and appearance but together are the full primarch to the legion. It is also stated that the Emperor knows of this; but whether he made it so or not is a mystery. The two primarchs supposedly share one soul, Omegon acting as Alpharius's second-in-command. The existence of two primarchs of the Alpha Legion is known only to the legion itself, and is a well-kept secret even from the other legions.
Alpharius, unlike the other Primarchs, was introduced to the Emperor in only a casual way before being given command of the XX Legion, ironically dubbed the Alpha Legion (whether this was out of irony for being the last Legion discovered or based on Alpharius's name is not known). Horus took Alpharius under his wing, and even gave overall command authority to Alpharius in later campaigns. In a bizarre turn, Alpharius refused to reveal the location of his homeworld, normally the site of much of a Legion's recruitment, to the budding Imperium. It is unknown whether Alpharius even had a homeworld.
The Alpha Legion's turn to Chaos was, oddly, something they chose for the Imperium's sake. While undertaking a compliance war on the planet of Nurth, they were approached by a man named John Grammaticus, an agent of an alien congregation known as the Cabal. The Cabal revealed to Alpharius and Omegon, as well as two other men, Peto Soneka and a psyker by the name of Shere, the outcome of Horus being named Warmaster. They then presented two options to the Primarchs. The Legion could either defend the Imperium in the upcoming war, and thereby doom the galaxy to ten thousand years of war, or they could side with Horus and condemn the human race to extinction but save the galaxy from Chaos.
the Cabal also shown to them the Effect if they sided with Horus; Horus kills the Emperor and a thousand human worlds die with him. After "two to three generations" (Legion, pg 393, Line 19) Horus is racked with guilt by slaying the Emperor and ultimately obliterates all Chaos-tainted factions and kills himself and the galaxy is purged from Chaos. Humans make the ultimate sacrifice. Then the Cabal showed him a preview of what was to happen if they sided with the Emperor; the Emperor's sacrifice, the corpse on the Golden throne, the billions upon untold billions of human souls devoured by Chaos, at this point, both primarchs staggered back and screamed at the vision. “The Alpha Legion has always, always, been for the Emperor,” Alpharius stated.
The Legion's battle cry of "For the Emperor!," when viewed in this light, is not the mocking insult it was once thought to be. Rather, it is a testament to the Legion's devotion that they have stayed true to their cause for so long. It is thought that the Alpha Legion's true goal is to release the Emperor from his "prison" which is the golden throne so the revelation made to them is fulfilled.
One interesting note is that most of the Alpha Legion does not reside in the warp, but rather roams the galaxy in bands of warriors, trained to act independently of each other toward their greater cause. In this way they are by and large the only Traitor Legion not to succumb to the mutations of Chaos.
The Alpha Legion has been metitioned in the Dawn of War series.

