There have been entire books devoted to the downfall of certain primarchs. Fulgrim, for example, saw the leader of the Emperor's Children's turn down a dark road. Betrayer saw Angron's potential fully unleashed. A Thousand Sons witnessed Magnus' tragic downfall. Legion turned Alpharius. And so on. An episode of herding cats, Slaves to Darkness (2018) brings this wild group back together as Horus sits on the doorstep of Terra.
Slaves to Darkness picks up in the direct aftermath of Wolfsbane. Horus lies wounded following Leman Rus' spear attack. In the void of leadership, Malaghurst attempts to keep the Sons of Horus ship upright, and like it or not, is faced with the task of getting the traitor forces organized. This includes coralling the wild primarchs Angron and Fulgrim. But Malaghurst's task becomes all the more complicated as the forces of Chaos rear their head(s). The assembly of forces anything but certain, the traitor forces may devour themselves before ever setting foot on Terra.