Horus scarcely needed to lift a hand on the battlefield.
There were people at his side to fight the war for him.
During the era of 30k, the aura surrounding Horus had endured for many long years. The moment Horus rebelled, several Legions responded directly, acknowledging his rebellion and following behind him.
Though this reflected the Emperor's attitude toward the Primarchs and their Legions at the time—an attitude that had indeed failed to make those Primarchs feel respected or loved—Horus's charisma was equally evident from it.
As those Chaos soldiers were slain, they clearly had not expected to be killed by their own officers, much less without any warning.
In an instant, four Black Legion Marauders turned on and slaughtered the Chaos soldiers around them, drawing the attention of the Chaos Daemons. A grotesquely mutated Daemon Engine that worked with Abaddon, Roarer, noticed the four traitorous Black Legionnaires.
Of course, there were also newcomers to the Black Legion who had no strong connection to Horus. Those recruits still obeyed the order established by Abaddon. They still listened to Abaddon's voice and followed in Abaddon's footsteps. Though Horus before them seemed vaguely familiar, the passage of time had long erased every impression of Horus from their genetic memory.
"Destroy the traitors!"
Two new Black Legion Marauders wore superb Daemon Terminator Power Armor forged by Vashtorr's dark Adeptus Mechanicus. Demonic sigils had even been branded into the fine equipment, allowing them to unleash extraordinary power in battle.
"Horus! You are Horus!?"
Under the powerful cover of the two Space Marines, a Chaos Daemon charged toward Horus. To him, this hulking figure looked so much like Abaddon, but that did not mean he possessed Abaddon's strength. And what did resemblance matter? He was immortal.
Even Abaddon could only banish him, not kill him.