Horus' approach to the heart of his ambitions started in The Solar War and landed on Terra in The Lost and the Damned. The assault on the Emperor's mountain fortress achieves a critical point in Gav Thorpe's The First Wall (2020).
Having made his way through the solar system, Horus stands at the doorstep of Terra. Below him, the Lion's Gate Spaceport sits as a meaty target. Take it over, and there is a path into the Emperor's fortress. But this is the one thing Rogal Dorne, primarch of the Imperial Fists, knows he cannot let happen. He has set up and organized defenses to repel what he thinks Horus will throw at him. In response, Horus gives Peturabo, his cleverest primarch, the task of taking the Spaceport. Let the battle begin.