Ok, feel like I missed a lot here, as I don't get some of the references, and I am up to date on the books
What is the significance of Horus at Molech (What/when/where is Molech?)
I thought Horus started losing as soon as his super-saiyan arrogance was ignited in the fever dream on Davin, or when he started listening to Erebus (where the hell has he been since book 2?) BUT, the Emperor did not help the issue, by unexplainedly withdrawing from his sons to create the opening in the webway that Maguns promptly fooked up in his hubris.
Had Woody only gone to the police......
Molech holds a doorway to the Warp, that the Emperor used to take the fight to the Ruinous Powers and in so doing, achieve his current level of divinity. Horus invaded that world basically to prove himself to his new masters, and to do about 10,000 years of power-leveling in the Warp. As for Erebus, he and Space Falwell and Space Graham were busy beating up Smurfs (Know No Fear, Mark of Calth), trying to kill the Emperor (Nemesis, Vulkan Lives), trying to keep the Angels out of the fight/turn Sanguinius (Fear to Tread) and turning Angron into the first officially recognized Daemon Prince (Betrayer, although arguments can be made both for Magnus, as the Scourging of Prospero happened around the same time as the battle of Isstvan IV, and Fulgrim, whose apotheosis occurred a few months after the Dropsite Massacre, in Angel Exterminatus).