Lost Primarchs

Overview
The narratives leave two additional unnamed and undescribed First Founding Legions, Legions II and XI. In some literature the missing Primarchs and their Legions are listed as being "Deleted from Imperial records". The only information Games Workshop has ever released that directly addresses them, can be found in False Gods and The Lightning Tower (Dan Abnett). This has given way to the theories that II could possibly be the founder of the Blood Ravens chapter

XI
In the Black Library Horus Heresy novel series, Horus, in a chaos-induced dream sequence, apparently goes back in time and sees the cracked incubation capsule of Primarch XI before the Primarchs are scattered to the warp (the effect this has on the Primarch is inconclusive). Before doing this, he puts his hand on XI's capsule and feels "the untapped glories that might have lain ahead for what grew within, but knowing that they would never come to pass".