Pic of Traitor Primarchs

Horus' vision

As Horus lay in coma inside the Davinite temple after being wounded by Eugen Temba, Erebus gave him visions that showed him events from various points in time. In one of these experiences, Horus found himself in the geno-vault where he was created, and looked down upon the genetor-tanks containing the infant primarchs. At one point he stops before tank XI and reflects upon the "untapped glories that lay within, knowing they would never come to pass". This refers to one of the unknown Primarchs. Later on, Horus' anger takes control and he smashes his fist upon tank XI, damaging it. Shortly after, a vortex swoops up all the genetor-tanks and scatters them across the galaxy.

The truth of this vision is unclear; it should be noted that the whole experience was orchestrated by the Chaos powers.14