1st Platoon proceeds from friendly lines and into enemy territory in search of General Constantine of the Severan Dominate. Signs of the rebels abandoning their posts and defensive positions point to a change in the situation on Lycurgos.
Alpha Squad is working their way down a corridor as part of the search and comes across twenty servitors busy disassembling a room and all of its contents, sorting the trash into bins and taking machinery and other parts to another chamber some minutes away. Sergeant Arion confronts one of the servitors to speak to it and learns that it is serving a Tech-Priest named Vilius. Checking in with command on Vox, he confirms that Vilius is not part of the Imperial forces. Following the line of servitors moving the parts, Alpha discovers that they are using the machinery to construct a hideous abomination born of Chaos - a Defiler!
The Defiler quickly spots the Imperial force and attacks with its flamer, the firey chemical projector injuring most of the squad. As Sergeant Arion calls for a retreat, Darrien Adel fires the APC's new autocannon at the daemon engine. Luckily for the squad, the Defiler cannot get past the doorway to pursue them, and while it tries to break past the wall the autocannon finally tears it into a million inanimate pieces. The squad calls command to inform them of the infernal enemy they have just destroyed and the servitors' involvement. They demolish two unfinished Defiler machines and Darrien begins purposefully running over any servitors they see after picking up the dismounts and driving on. Over the vox, the squad can hear that other platoons are also encountering heretical forces serving Chaos.
The troopers next come across a storage facility and dismount from the Chimera to search it for signs of Constantine. Adel's sharp eyes notice the shadow of a hiding Chaos militiaman in the room's catwalks, and he alerts the others by firing his autocannon at the would-be ambush. The squad pulls back to the APC and the scarred and mad militia force pursues them. Yekaterina calls on her psychic power to wreck a ladder being used by the chasing soldiers, killing many of them. Unfortunately she also causes whispers from the Warp to descend upon the scene. While Alpha mostly resists the terror they feel, the psyker herself begins to flee in a panic.
When a renegade Chaos psyker appears with reinforcements to support the decimated ambush squads, Sergeant Arion coordinates the squad to send volleys of lasgun fire into them. While the enemy force is slaughtered, the psyker uses its evil power to suck some life force from Arion. Once the battle is over, Isabella catches up to Yekaterina and the psyker calms down, giving herself some ritual scars to atone for her panic.
Alpha presses on and soon sees a large group of seemingly mundane servitors ahead. They suddenly unleash a trap that drops an entire section of dismantled bulkhead toward the APC, forcing Adel to swerve out of the way avoid getting stuck. The servitors' deadly weapon attachments and berserk charge at the Chimera reveals that they are actually murder servitors, and while most of them are gunned down by the vehicle crew and Sergeant Arion manning a mounted storm bolter, one is able to clamber on top of the speeding transport. Arion blows it apart at nearly point blank range, then calls in to update command. Word soon comes down from above that all servitors are to be presumed to be hostile and should be destroyed if encountered. Alpha is told that they are at the advance of the move toward the central core, and reinforcements are directed to back them up.
Arriving in the vicinity of the command deck at the heart of the star fort, the squad sees more signs of traitors forces abandoning their defenses. Moving up to a door, Sergeant Arion sees that the massive command chamber is the scene of a fierce engagement between a large force of Severan Dominate soldiers and Chaos cultists who are moving past a massive and deadly Maulerfiend. Among the Dominate units is an officer who Arion identifies as General Constantine, their primary objective. Constantine is commanding his troops to take down the Maulerfiend, but their weapons are having little impact on it. Arion and the dismounted infantry of Alpha get into position to target the cultists and the APC aims into the room from the rear to target the terrible fiend.
As the Chaos cultists charge across the command deck to engage the Imperial Guard force, Arion coordinates fire on the them and the Chimera levels its heavy guns on the Maulerfiend. While the cultists are no match for the lasgun volleys, the Maulerfiend proves a hardier enemy for the APC's weapons, taking many hits from the autocannon before it finally collapses, having begun to rush the Severan Dominate force. Arion considers attacking the rebels to enable the squad to more easily capture the General, but he sees that there are two full squads of shock troops and five squads of standard infantry remaining; Alpha might still win the engagement through skill and valor, but it would be at a very high cost.
As Arion confronts General Constantine and tries to intimidate him into giving himself up, the latter makes it clear that he has no intention of simply turning himself over to the Imperials.