Omegon
The twin brother of Alpharius; collectively, they are the Primarchs of the Alpha Legion. People know less about this guy than they do about Alpharius, so that's pretty much less than jack squat. In fact, people know so little about this guy and his brother that it isn't even known if they are one person or two. This is leveraged by the fact that everyone is either one of these guys or the other, including themselves.
If you don't know anybody who is secretly Alpharius or Omegon, you probably are him (or the other one (or both (or neither(or all at the same time)))).
...
Fucking samefag.
He was the one who invented the tyranids. They are actually a fiendish destraction so that he can break into the imperial palace on terra and raise TEH EMPRA from the dead.
Conspiracies within Conspiracies (as usual)
Consider the following evidence:
1. Alpharius/Omegon sided with Horus despite being explicitly told by The Cabal prior to the Heresy that if Horus won it would mean the end of the human race but the ultimate victory against Chaos. While if the Emperor won it would mean the decay and eventual death of the galaxy, but humanity survives.
- (Catch 22 with two undesirable outcomes, but hey, the Alpha Legion has two Primarchs, go figure?)
2a. Omegon discretely plotted against Alpharius to destroy an Alpha Legion facility which was causing warp storms and preventing the White Scars from returning to Terra. If this facility was not destroyed the White Scars would not have been present during the final battle.
- (One brother conspiring against the other? Just like the rest of the Horus Heresy. Just because they are twins it doesn't mean they are always of the same mindset)
2b. When the White Scars did eventually leave the Chondax system, the Alpha Legion "fleet" (actually made up of several "fake" battlecruisers with no real teeth) arrived to hold them in position long enough to recieve orders from Rogal Dorn to return to Terra.
3. Someone in the Alpha Legion saved Corax and the survivors of Istvaan from the World Eaters
- If Corax died, the Raven Guard would have been finished and out of the war for good (much like the Salamanders were) instead they went on to rebuild and rejoin the war towards the end and still have enough marines left over to split into successor chapters
4a. Despite the Alpha Legion stealing the ORIGINAL genetic template of the Primarchs, they handed false information to Horus which would never have resulted in anything useful.
4b. ...said stolen genetic information was never utilised by the Alpha Legion either, even after 10,000 years, even when they could have been producing "super" space marines!
- ...hey? aren't Grey Knights an "original" space marine chapter with NO known primarch? with psychic powers? AND an unusual resistance to corruption? And supposedly improved genetic template?
5. Omegon kept a hidden suit of armour on board his battle barge in addition to his "normal" legionary armour, one which he had planned a hidden purpose for to "strike unexpectedly like a serpent". This other suit was plain and unadorned...
- ...unpainted power armour is grey, also it's what Malcador's knight-errants wore.
6. The first Grand Master of the Grey Knights was named "Janus"
- the Roman god with two faces of "beginnings & endings" ie: Alpha & Omega
Let's just pray to the Emperor that this is not all just coincidence
| The Primarchs of the Space Marine Legions |
|---|
 Loyalist Corvus Corax - Ferrus Manus - Jaghatai Khan Leman Russ - Lion El'Jonson - Roboute Guilliman Rogal Dorn - Sanguinius - Vulkan |
 Traitor Alpharius/Omegon - Angron - Fulgrim Horus - Konrad Curze/Night Haunter - Lorgar Magnus the Red - Mortarion - Perturabo |