Alpha Legion
No such legion exists, nor has it ever existed. Perhaps you were looking for the main page?
The term is used by the heretical to refer to a mythical 20th Legion. It is nonsensical name, which proves it is a false story, since true priests of the Adeptus Ministorum would know 'alpha' is an ancient term meaning "first," but any such 20th Legion would have been the last to be discovered before the Horus Heresy. Adherents to this story make addle-minded claims that prove their insincerity, such as all marines in this Legion were surgically altered to look identical to the primarch, or that the Emperor himself gave this primarch command over campaign forces above that of the Ecclesiarch, even power fantasies such the existence of more than one primarch in this Legion, and other tall tales that defy belief. Most heretical of all is the claims that these primarchs betrayed his holiness the Emperor of Mankind willingly without being corrupted by Chaos, which conveniently excuses their absence from the Eye of Terror and thus any observable evidence of their betrayal with the other traitor Legions.
These stories are obvious and self-contradictory falsehoods, trash that pollutes the minds of the faithful. We apologize for any discomfort this wiki article may have caused you -- speak to your local chaplain or commissar if you require counseling. If you witness anyone spreading these harmful lies about a 20th Legion, immediately report the heretic to your local representative of the Inquisition or to your unit's commissar.
There are extremely persistent and consistent rumors that the most blessed Blood Ravens chapter of the holy Adeptus Astartes lead by Force Commander Gabriel Angelos and assisted by the Librarian Isador Akios and the PDF and Imperial Guard lead by...someone... and the Foul Xenos of the Eldar craftworld Biel'tan lead by Farseer Macha fought the mythical Alpha Legion lead by Chaos Lord Bale and Chaos Sorcerer Sindri Myr and an Orkish Waaagh! lead by the Warboss Orkamungus on the planet Tartarus (what a lovely name for a planet). Attempting to accuse the Blood Ravens of Heresy for spreading lies about fictional legions has universally ended in failure.
Also any relation between the Alpha legion and Tzeentch dogma is purely coincidental. But everyone agrees they are space terrorists, and probably al-Qaeda with a grimdark-startrek-feel.