In short, when Horus was loyal he had 4 Space Marines who acted as advisors, Abaddon, Tarik, Loken and Little Horus. Loken and Tarik remained loyal and were attacked at the beginning of the Heresy on the planet Istvan III, Horus sent Abaddon and Little Horus to kill Loken and Tarik, Little Horus killed Tarik in front of Loken's eyes, this fact It traumatized Little Horus and filled Loken with rage but Abaddon "almost" killed Loken before he did anything. During the Heresy, Little Horus felt that something was stalking him, then that something was Loken but a product of his madness. During the siege of Terra, Little Horus encounters Loken, Little Horus thinks it was a hallucination but it turns out it was the real Loken who tears him apart and avenges Tarik's death.