Lost Primarchs

The Lost Primarchs of the First Founding Space Marine Legions are the two Primarchs of the II and XI Legions who, for unknown reasons, were deliberately expunged from all known Imperial records and archives before the onset of the Horus Heresy in the early 31st Millennium. Referred to as "the forgotten and the purged" it is known only that the missing Primarchs and their Legions are listed as having been "deleted from Imperial records." This formal censure and erasure from official records is known as an Edict of Obliteration, also called a Damnatio Memoriae, a High Gothic phrase meaning "condemnation of memory." This is the official Imperial policy of deliberately destroying any records, icons or other symbols or monuments pertaining to an individual or organisation, usually of the Imperial elite, who has been declared Excommunicate Traitoris by the Emperor of Mankind (before he was interred within the Golden Throne), the High Lords of Terra or the Inquisition. In a galaxy-spanning empire that stresses fealty and loyalty to the Emperor in return for advancement, acclaim and spiritual salvation for its elites, this is perhaps one of the most severe punishments. The complete and utter erasure of all records of the II and XI Legions is considered by Imperial historians as the most successful Edict of Obliteration ever carried out in Imperial history.

History
In the short story The Lightning Tower, when the Imperial Fists' Primarch, Rogal Dorn, is in the falls of the Kath Mandau Precinct of the Imperial Palace and comes upon a series of pillars with plinths that contain life-sized statues of all the Space Marine Legions' Primarchs:

"The second and eleventh plinths had been vacant for a long time. No one ever spoke of those two absent brothers. Their separate tragedies had seemed like aberrations. Had they, in fact, been warnings that no one had heeded?" - Primarch Rogal Dorn, in the falls of the Kath Mandau Precinct - Imperial Palace, Terra

Another reference referring to the lost Primarchs:

"...in the destiny it would shape for their species. They had all believed. All of them." - Primarch Rogal Dorn, in the falls of the Kath Mandau Precinct - Imperial Palace, Terra

Earlier, less ambiguous sources go so far as to say that information on the lost Legions and Primarchs was deleted following the Horus Heresy. The only information Games Workshop has ever released on this subject that directly addresses the mystery can be found in the Horus Heresy novels False Gods and The Lightning Tower by Dan Abnett. This has given way to the theory that Primarch II could possibly be the founder of the Blood Ravens Chapter. This theory, however, gives undue importance to the Blood Ravens, because, outside of the Dawn of War PC games, they are in no way a significant Chapter in the Imperium of Man's history. This theory also does not describe why the Blood Ravens would still be around in the 41st Millennium - obviously their forebears had committed some terrible acts of treachery for their records to be completely destroyed, to such an extent that even other Legions present at the time presumably also had any of their own records of the lost Legions and their Primarchs destroyed or modified.

Games Workshop has shed a little more light on this subject in the Horus Heresy novel about the Word Bearers called First Heretic. It has been established that these "missing" Primarchs were erased from Imperial records at least 43 years prior to the events that would occur on Istvaan V, as only 18 of the established Legions are referenced during the events that occurred at Monarchia on the world of Khur, where the entire Word Bearers Legion and their Primarch Lorgar were formally rebuked by the Emperor of Mankind for their apparent laxity in discharging their duties of bringing the atheistic Imperial Truth to newly Compliant human worlds during the Great Crusade.

"I fear the Emperor will break the Word Bearers – and break me. We would be cast alongside the brothers we no longer speak of." - Lorgar, Primarch of the Word Bearers talking to Magnus the Red, Primarch of the Thousand Sons

There is also a long standing rumour amongst the Astartes Legions that is revealed that may explain what the ultimate fate of the II and XI Legions was. Around the time that both these wayward Legions were expunged from all Imperial records, it was said that the number of the Astartes in the Ultramarines Legion swelled in size until they eclipsed all of the other Legions. It is not known if this is merely speculation or it is established fact that the II and XI Legions of Space Marines were inducted into other Legions after they were officially expunged from Imperial records. Though interesting, this still does not explain what actually happened to the Primarchs of the II and XI Legions:

"But the Eleventh Legion -"

"Is expunged from Imperial record for good reason. As is the Second. I'm not saying I don't feel temptation creeping over me, brother. A single sword thrust piercing that pod, and we'd unwrite a shameful future."

Dagotal cleared his throat. "And deny the Ultramarines a significant boost in recruitment numbers.

Xaphen regarded him with emotionless eyes, seeming to weigh the merit of such a thing.

"What?" Dagotal asked the others. "You were thinking it, too. It's no secret."

"''Those are just rumours," Torgal grunted. The assault sergeant didn't sound particularly certain. ''

" Perhaps, perhaps not. The Thirteenth definitely swelled to eclipse all the other Legions around the time the Second and Eleventh were "forgotten" by Imperial archives." - A conversation between Word Bearers of the Gal Vobak of the Serrated Sun Chapter

We are further illuminated on the subject in the novels Legion and Prospero Burns. In the novel Legion it is mentioned that a Legion of the Legiones Astartes had gotten in trouble before:

"I believe their operation and conduct should be reported to the Council of Terra, pending censure or dissolution. It wouldn't be the first time a Legion Astartes has overstepped the mark, after all." - Chayne talking to Namatijira

In the novel Prospero Burns the Space Wolves Primarch Leman Russ discusses the inevitable execution of the Thousand Sons for Magnus the Red's performance of an act of maleficarum that drove his sorcery right into the heart of Terra and into the presence of the Emperor, revealing his own supposed treachery through defiance of the Imperial edicts banning the use of psychic sorcery at the Council of Nikaea. When asked if he was concerned over the fact that the unprecedented was about to take place, namely, Astartes fighting Astartes, Russ commented that this was not an unprecedented event, implying that his Wolves had been called on to perform this task once before. This might be an enigmatic reference to the possible destruction of the II and XI Legions, or it could be reference to the little known event, known as the Night of the Wolf as mentioned in the novel Betrayer. This was a little known incident that occurred shortly after the massacre of the entire planetary population of Ghenna. The Primarch Leman Russ had been charged by the Emperor to take his Space Wolves Legion to Ghenna to bring the World Eaters to heel:

(Kasper Hawser) "There's a first time for everything''." (Russ)"Exactly," he grunts.

"The unprecedented. Like… Astartes fighting Astartes? Like the Rout being called to sanction another Legion?"

"That?" he answers. He laughs, but it is a sad sound. "Hjolda, no. That's not unprecedented." - Primarch Leman Russ speaking to the Skald Kasper Hawser

In the novel The Outcast Dead, the character Evander Gregoras, the Master of the Cryptaesthesians, remarks that because of Magnus' violation of the Emperor's Decree Absolute from the Council of Nikaea for performing an act of maleficarum, the Emperor would have no choice but to make an example of his wayward son and the Thousand Sons Legion:

"It will mean the Wolves will be loosed again." Evander Gregoras talking to Kai Zulane

This statement further vindicates the role of the Space Wolves Legion as the Emperor's chosen executioners -- to enact His will when carrying out censure against a fellow Space Marine Legion.

In the novel Prospero Burns, the character Kasper Hawser also states:

"A Legion is always strongest at its base. The fortress homes of the Allfather’s eighteen Legions Astartes are the most formidable and impregnable sites in the new Imperium." - From Kasper Hawser's account of the Burning of Prospero

This statement implies that the missing II and XI Legions were already gone prior to the Burning of Prospero, and thus, prior to the Horus Heresy.

We are further illuminated in the novel Deliverence Lost, where the Primarch Corvus Corax of the Raven Guard recalls his first meeting with his father, the Emperor:

(Emperor) "You and your brothers were taken from me by denizens of the warp before you were ready."

"Brothers?" Corvus was excited by the prospect, pushing aside the questions that the Emperor's answer had prompted. Though he had made many friends amongst the prisoners of Lycaeus, always Corvus had been aware of his otherness, and when they had started to call him Saviour any hope of normal relationships had ended. That there were others like him filled Corvus with hope again.

"Yes, you have brothers," said the Emperor, smiling at his son's delight.

"Seventeen of them. You are the primarchs, my finest creations."

"Seventeen?" Corvus asked, confused. "I remember that I was number nineteen. How can that be so?"

The Emperor's expression grew bleak, filled with deep sorrow. He looked away as he replied.

" The other two," he said. "That is a conversation for another day."

This revelation indicates that the missing II and XI Legion were already lost and possibly expunged from all Imperial records by the time Corax was reunited with the Emperor on Lycaeus. This conversation also implies that the Lost Primarchs were more than likely lost in the early years of the Great Crusade, as the Emperor mentions that most of the other Primarchs had already been discovered by the time he reunited with Corax.

Legion/Primarch II
In The Lightning Tower by Dan Abnett, while constructing the defenses of the Imperial Palace on Terra in preparation for the coming attack by the Traitor Legions, Primarch Rogal Dorn of the Imperial Fists Legion is walking through the Palace and comes across a corridor showing statues of all 20 Primarchs. The audio book states that "an accident befell them" that may somehow be a precursor of what happened to Horus. In the novel Mechanicum a conversation between Imperial Fists Primarch Rogal Dorn and Malcador the Sigillite in regards to the war on Marsand the Heresy:

(Malcador)"... Horus has three of his brother legions with him, you have your fists and thirteen others."

"Would that it were fifteen,' mused Dorn

"Do not even think it, my friend,' warned Malcador. 'They are lost to us forever."

I know'',' said Dorn.

- A conversation between Primarch Rogal Dorn and Malcador the Sigillite

In any case, it is unlikely that the current writers at Games Workshop could do justice to the now much anticipated mystery of what happened to the Lost Legions and why they were deleted from the Imperium's historical records.

Legion/Primarch XI
Following his terrible wounding on the moon of the world of Davin by the Chaotic weapon known as the Kinebrach Anathamae, the Primarch Horus, in a Chaos-induced vision, apparently went back in time to the Emperor's gene-laboratory beneath the Himalayan Mountains and saw the incubation capsule of Primarch XI before the Primarchs were scattered through the Warp (the effect this has on the Lost Primarch is unknown). Before the Warp Rift opened within the lab, Horus put his hand on Primarch XI's capsule and felt it:

"He stopped by the tank with XI stenciled upon it ... feeling the untapped glories that might have lain ahead for what grew within, but knowing that they would never come to pass." - Horus, during a vision granted to him by Chaos while recovering from wounds received on Davin's moon

Purpose
However, Games Workshop has explained the game play purpose for these "deleted" Legions: they are available to help those who wish to develop their own, custom Astartes Legions. The idea is that because there were 20 Space Marine Legions created by the Emperor, and half turned to Chaos but half stayed Loyal, these two lost Legions would represent equivalent sources for new custom-made Space Marine Chapters and Chaos Space Marines warbands.