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The last [[Space Marine Legion|Legion]] created during the [[First Founding]], work on the XXth Legion was begun only some few decades before the discovery of their Primarch, [[Alpharius]]. However, other reports exist claim that the Legion existed in a prototype form long before that.{{Fn|17}} Indeed, [[Remembrancers]] seem to give contradictory reports on when exactly the Alpha Legion was formed.{{Fn|17}} Regardless, it seems the Legion was created in total secrecy on the [[Emperor of Mankind|Emperor]]'s order. Information on the Legion's own [[Gene-seed]] was made top secret. Like the [[Salamanders]] and [[Space Wolves]], the XXth Legion was formed and established largely in separation from the rest of the [[Legiones Astartes]], likely for a very specific purpose. These three specific legions were obliquely called "[[Trefoil]]" by some sources .{{Fn|23a}} The earliest known activities of the Legion saw it conduct abductions, targeted strikes, and assassinations of the [[Imperium]]'s enemies on [[Terra]] and beyond. The designation for this mysterious special operations units, taking orders from none of the [[Primarch]]s who had been discovered at the time, was simply "Alpha". For reasons unknown , approval to rapidly expand the Alpha Units gene-seed group was denied by the [[Emperor of Mankind|Emperor]], limiting the force to only around 1,000-2,000 Legionaries. There is some suspicion that this was done because there was a potential problem or flaw within the Legion's gene-seed that the Emperor was not able to rectify. Others speculate that the Legion was deliberately preserved separately as an isolated unit by the Emperor for special gene-seed augmentation in the future that no other Space Marine Legion would ever undergo.{{Fn|23a}}

The [[Space Marine Legion|Legion]] all but disappeared from [[Imperium of Mankind|Imperial]] records in the [[Great Crusade]], but some reports of small Alpha units operating far from the front lines of the [[Great Crusade]] in special missions do exist. These warriors were clearly [[Space Marines|Astartes]], but carried no heraldry and sometimes even operated under the 'false flag' of another Legion. The operations they undertook were assassinations, subterfuge, espionage, sabotage, and the recovery of unknown artifacts. They also abducted important individuals from anti-Imperial groups. They made it a policy to never leave any witnesses to their activities. However, rumors rumours still spread across the galaxy Galaxy of a ''Ghost Legion''. This force became known to bleed enemy forces through black operations and subterfuge before striking from a hundred directions in a mass lightning attack known as ''The Harrowing''.{{Fn|23a}} A dubious account given by [[Alpharius]] himself states that he was in fact recovered by the Emperor on [[Terra]] himself shortly after the scattering of the Primarchs, and was put in command of his Ghost Legion from its onset. During this time, his existence was kept a total secret from all save the Emperor, [[Malcador]], and later [[Constantin Valdor]].{{Fn|66a}}

During the [[Rangdan Xenocides]], a representative of the XXth Legion appeared before [[Lion El'Jonson]]. Though identifying himself as [[Alpharius]], the representative acknowledged that their [[Primarch]] had not yet been found but that he would be in time. This ''Alpharius'' (who in a contradictory account, may have in fact been the actual Alpharius{{Fn|66c}}) offered aid to Lion El'Jonson against the [[Rangdan]] in the hope that one day The Lion could be made [[Warmaster]]. Another account states that Alpharius used this objective as cover as a way to make his way into the Rangdan warzone to recover his longlost brother [[Omegon]].{{Fn|66b}} Whatever the truth, this Alpharius revealed that the XXth Legion preferred El'Jonson as Warmaster over someone like [[Roboute Guilliman]] as they shared similar views on war and secrecy. Though a Primarch, not even El 'Jonson had been explicitly aware of the XXth's existence, though he had been privy to rumors rumours of a "Ghost Legion".{{Fn|35}}

Later when Alpharius-Omegon took open command of the [[Space Marine Legion|Legion]], it was young, zealous and completely committed to embracing the [[Primarch]]'s directions. [[Alpharius]] believed that secrecy and fluidity brought success, and taught his Legion to apply all such military techniques to both their training and their operations. The Legion's victories in the [[Great Crusade]] all feature some form of subterfuge, misdirection or rapid, unexpected movement. Such victories required great skill and dedication to achieve, and the Alpha Legion quickly became an insular and proud formation.{{Fn|1a}} It was this martial pride that led to a number of clashes and fights with members of the other Legions, particularly the [[Imperial Fists]], [[Ultramarines]], and [[Death Guard]].{{Fn|7b}}{{Fn|17}} Also, [[Konrad Curze]] of the [[Night Lords]] openly criticized criticised the Alpha Legion's tactics by calling them "sinners in a shroud of lies".{{Fn|17}} The [[Thousand Sons]] Legion discreetly shunned and tried to avoid any contact with the Alpha Legion, although the true cause for this is unknown.{{Fn|17}} However, at the side of [[Horus]]' [[Luna Wolves]] the Alpha Legion successfully campaigned without incident , together with the [[Dark Angels]] and the [[Iron Hands]].{{Fn|17}}

After Alpharius' disagreements with [[Roboute Guilliman]], the Alpha Legion threw themselves even further into their preferred method of operations, largely cutting themselves off from standard [[Imperium|Imperial]] practices and orchestrating greater and greater victorious examples of their approach to the [[Great Crusade]], even when more conventional attacks would have been more efficient. The most notorious example of this took place on the world of [[Tesstra Prime]], where the Alpha Legion, instead of taking the opportunity to capture the planetary capital to force the world's surrender, allowed the enemy to dig in and defend it so that they could then expertly take the defending forces apart in a number of different ways. After a week of suffering seemingly random mishaps as well as brutal ambushes, the defenders were forced to capitulate, having taken 90% casualties. When asked why the [[Space Marine Legion|Legion]] had not taken the simpler strategy, [[Alpharius]] is reported to have replied that they avoided it as "it would have been too easy". This campaign brought him censure from almost all of his brother [[Primarch]]sPrimarchs; only [[Horus]], always impressed by Alpharius and his work, praised the Alpha Legion's skill.{{Fn|1a}} In the later stages of the Great Crusade, the Alpha Legion withdrew into the empty space on the Imperium's frontiers.{{Fn|17}}

With such examples existing in the [[Imperium|Imperial]] records, it is perhaps easy to see why the Alpha Legion sided with [[Horus]] when the [[Warmaster]] made his pact with [[Chaos]]; their martial pride and [[Alpharius]]' avoidance of all his brothers apart from Horus seemingly led to their downfall. However, another given reason is that, a scant two years before the [[Horus Heresy]] began, [[Alpharius Omegon]] was contacted by a [[Xenos]] organisation known as the [[Cabal]], which presented the [[Primarch]] with visions of the Heresy to come and other predictions of the future as well as knowledge about the nature of Chaos. They were shown that the only outcomes of the Heresy were that, if the [[Emperor of Mankind|Emperor]] won, humanityHumanity's existence would be ensured for ten or twenty thousand years of decay before they and the galaxy Galaxy were consumed by Chaos and that, if Horus won, humanity Humanity would perish inside two generations, taking the Chaos powers into oblivion with them, thus saving the rest of the galaxyGalaxy. The Alpha Legion was asked to take on their greatest challenge; to defect to the side of Horus and ensure the final destruction of Chaos. Alpharius Omegon appeared to accede to this request.{{Fn|2}}