The default portrayal is set during the latter Horus Heresy, after the Luna Wolves have become the Sons of Horus but before the Saturnine assault ends Falkus Kibre’s first recorded life.

The Legion has already turned traitor.

Horus has been wounded, transformed, and increasingly removed from ordinary command by the powers surrounding him. The Sons of Horus remain the Warmaster’s own Legion, yet their unity has begun to rot beneath factional rivalry, sorcery, ambition, and the fear that their father is becoming something none of them can truly understand.