Isstvan III Atrocity - The Siren Hold: Across the city, with the sundering force of the claw-strike of a great predatory beast, the Sons of Horus Drop Pod assault descended upon the Siren Hold. Larger than the precincts of the Precentor's Palace, the Siren Hold was a scatter of great irregular conical tomb-spires, in appearance almost like deep ocean smoke-stacks, jutting hundreds of metres into the air. Scattered and under heavy fire from the outset, the Sons of Horus nevertheless rallied to the attack with their customary discipline and battle-lust, and the Siren Hold quickly became a tortuous, multi-storey killing ground, filled with shrapnel and smoke. It was here the Isstvanian Warsingers made their first appearance in strength. These psyker-witches were able to manipulate sound by the means of strange relic-technology to take to the air and surround themselves with bubbles of shielding sonic force. Their screaming songs were able to shatter the ceramite armour of the Space Marines and rupture the flesh and powder the bone within, and drove the warrior-cultists that defended them to inhuman levels of fanaticism so that they hurled themselves at the Sons of Horus heedless of the shredding Bolter fire that met them.As the battle was fully joined, the death toll mounted staggeringly as a great paean of howling sound engulfed the Choral City, drowning out even the din and cacophony of battle -- an unnatural scream that tore at the brain and sent the people of Isstvan III into shuddering paroxysms of uncontrolled hatred. It was an auditory barrage that only the Space Marines' willpower and auto-sense-baffled armour allowed them to resist and fight on, regardless of the strange assault. At the western walls and trench works the Death Guard and the Legio Mortis had left the great bulwarks in rubble and now the Legionaries were brutally sweeping the bunkers and tunnel networks beneath clear of defenders, while in the plaza area the World Eaters were surrounded by a sea of slaughter. Whatever dark power drove the terrified populace suddenly drove them forwards towards their attackers, heedless of their lives as they came at the World Eaters, first in dozens, then in the hundreds and thousands. Unarmed and unarmoured, the screaming, laughing human waves recklessly came to Angron's already blood-spattered warriors to die, and the World Eaters were keen to oblige. As wave after wave came at the World Eaters they were pushed back by the simple overwhelming weight of human numbers. Warriors fought back-to-back, sending Frag Grenades into the mass of densely packed bodies and hacking and slaying until the ramparts were covered with dripping meat. The XIIth Legion's heavy support squads ran their weapons dry cutting great reaping swathes through the onrushing crowds with their Heavy Bolters and Missile Launchers.In the tangled warrens of the Siren Hold and the inner walls of the Precentor's Palace, victory was being clawed from the enemy a metre at a time, but not without cost. The unnatural psyker-powers of the Warsingers slew many before they were brought down, and the elite warriors of the rebellion's inner-sanctum were encountered -- surgically modified and mutilated, clad in black vitrified armour and bearing strange and hitherto unknown relic weapons that spat killing blasts of sound or darts of liquid metal. Deep within the Tomb Spires, the Sons of Horus spearhead, under the command of senior Captain Garviel Loken, fought their way into a strange, corpse-filled shrine of foul idolatry, later named Death's Tomb, and slew the Warsingers there, while at roughly the same time, although having suffered heavy casualties, a small strike group of Emperor's Children under the command of the famed swordsman Captain Lucius fought their way into Vardus Praal's throne room and confronted the traitor.Praal, armoured in twisted baroque splendour and wielding strange psycho-sonic weaponry, was indeed the master of this strange and arcane cult rebellion, but even his augmented might was not enough to best Captain Lucius, who slew him in desperate battle. Regardless of whether Praal's death served to cut the head off the serpent, or if the destruction of the vile shrine at the centre of the Siren Hold was its cause, the strange cacophony that had held sway over the city collapsed with Praal's death and with it the power that had maintained the Isstvanians' resistance was ended. Within hours the rebellion was in tatters with all of the invasion force's objectives secured and tens of thousands of the enemy dead in the wake of the assault. The victorious Space Marines were jubilant, their victory hard-won and well-deserved, but unbeknownst to them, disaster was about to strike.