Editing The Assault on Ostium

The Thunder Kings, though not present at Isstvan, were present at a very early battle, the Assault on Ostium. The Emperor was enraged by the fracturing of the Terran Webway gate, but the thought of simply sending a force to execute his son was more than he could bear to think. The Voidwatcher would be brought back to Terra, so that his father could look him in the eye and hear the reasons for his treachery. For this purpose, the Emperor called in both Golgothos, whose heavily armed Entombed would be ideal for an operation on a collapsing planetoid like Ositum, and Brennus, whose legion was one of the largest still loyal to the Throne. The two rendezvoused at the Entombed's homeworld of Sepulchra, where Brennus offered to provide runic charms against sorcery for his brother. Golgothos and his sons readily accepted, and Brennus and his Runesmiths spent the transit to Ostium working these mystic sigils into The Entombed's armor. Brennus also presented his brother with a mighty hammer he had named “Warlock's Dread”.
 
Brennus and Golgothos, while working together in the forges, created a plan for the taking of The Black Augurs homeworld. Brennus and the Thunder Kings would establish beachheads on the world, forcing the Black Augurs to retreat at least some distance, and then the Entombed would teleport in from space, hitting those areas where the Black Augurs were concentrated most heavily to keep them from overwhelming the Kings with summoned daemons, and to hopefully shatter their strongest positions while the Thunder Kings kept reinforcements from arriving. Brennus attempted to convince his brother to stay in the rear guard, and perhaps support his sons at range, but Golgothos would hear nothing of that; he ached to get close to the traitorous sorcerers and teach them their folly. Brennus relented, but instead sent a detachment of the Horned Gods to fight alongside his brother, along with Golgothos' elite guard, The Death Masks. At first, this plan worked quite well: the Thunder Kings were able to hold key fronts long enough for the Entombed to enter the fray. In one notable case, an Entombed Dreadnought pod crushed a sorcerer as he was preparing a spell to slaughter a whole squad of Thunder Kings who had been paralyzed by warp-lightning. Across the battlefield the arrival of The Entombed would spell disaster for the Black Augurs, the heavy hitting, heavily armored warriors pulverizing warlocks across the battlefield. Golgothos was on the front lines, exacting a fearsome toll on the traitors of Istvaan V. Unfortunately, the terms of battle would not favor the loyalists for long.
 
Unbeknownst to the Thunder Kings and The Entombed, events were spiralling out of control beyond their reach. Arelex Orannis, Primarch of the War Scribes, was devastated by the treachery of Istvaan. As his legion had suffered severe casualties, he began to withdraw his troops from across the galaxy, so as to concentrate the remaining loyalist forces and prevent any of his men from being wiped out piecemeal. This action unfortunately allowed a hidden base of Life Bringers the opportunity to sneak free of the Scribes with their deadly payload: a modified form of the Life-Eater virus, specially altered to work only on being sharing the geneseed of The Entombed. The spies within the Thunder Kings, meant to sow discontent, quickly picked up on the objective of the joint mission. They funneled the information to their masters in the Children of Armok, who supplied it to the Life Bringers. Armed with this information, the dark healers of Vrachs' legion would make for Ostium with all speed, eager to unleash their twisted science on what remained of Golgothos' army.
 
As the battle raged on, the Black Augur's daemon summoning would grow to be a problem for the loyalists; even the protective runes of the Thunder Kings could not prevent the horrors of the warp from taking lives for their masters. Brennus, who had remained behind the front lines to better direct the combined loyalist forces, sent Golgothos and a detachment of his finest warriors, along with several bands of the 5th grand company Terminators, to strike at a fortified position from where the majority of the daemons had been loosed. This act would unknowingly save The Entombed from total annihilation; shortly after Golgothos' force made their initial attack on the Augur's summoners, Thunder Kings on the outskirts of the battle reported that a mysterious red fog had begun to roll in. At first, this was ignored; these furthest positions consisted of only Thunder Kings outriders and scouts, who assumed that it was simply an attempt by the Black Augurs to obscure the field of battle and adjusted accordingly. Oscara mac Damman, one of the Horned Gods and a legion company commander, was leading an assault in joint command with a member of the Death Masks, a nameless warrior known only as The Butcher of Kaelor. When the red fog began to creep up on them, Oscara watched it closely, knowing that there had to be something more than a simply obscuring effect; the Thunder Kings were hardly unprepared to pour a storm of fire into the fog, if so needed. But when the fog finally reached the first of The Entombed, he understood its purpose with complete horror: the normally stolid and silent men began to shriek in pain, as flesh began to melt away from their bodies. Oscara reported this to Brennus immediately, as he tried to command his men to retreat and get The Entombed off the field as quickly as possible; sadly, he was struck down by plague rounds from the fog, as the Life Bringers fired upon the now vulnerable Thunder Kings.
 
Reports quickly spread throughout the loyalist assault group; Entombed were dying across the moon, their horrific screams choking the vox and disorienting their allies. The Thunder Kings fared no better, as without their heavy support they were quickly overwhelmed by the combined might of plague horrors, daemons, and the assault of two Chaos-tainted Legions. Brennus ordered his men to retreat at all speed, and to save as many of The Entombed if possible; few of either side survived however, only 20,000 of the Thunder Kings original 60,000 strong force making it out of the mist, and only a few hundred of the Entombed able to escape from the mist. Golgothos and the marines he had fought with made up the bulk of The Entombed survivors, about 10,000 as the position they were assaulting was isolated from Life Bringers support and thus the fog. Brennus pleaded with Golgothos to retreat, but the screams of his sons had driven Golgothos battle-mad, and he would countenance no action besides an immediate slaughter of the Life Bringers. Brennus raced to confront Golgothos, and attempt to convince him, but the Tomb Lord's mighty dreadnought form quickly knocked his brother aside. Unwilling to allow another of his brothers to die, he struck Golgothos with a rune-crafted sling bullet, which pulsed with electricity and overloaded the dreadnought's servos. Brennus instructed the remaining warriors to escort Golgothos off the planet, as he supervised the rest of the retreat; he would remark later that "the screams of The Entombed were all the more terrible for their customary silence, and will haunt me until the last of my days. But the screams of their father, his pain and loss, will echo for millennia."