Battle of Calth - Miracle of Miracles: It fell to Chapter Master Empion to formulate and oversee a daring counterboarding assault, drawing on every Ultramarines Legionary his staff could muster. Forty groups, each numbering up to thirty Space Marines, passed through the armoured air gates and advanced on their designated targets. Each was assigned a vital objective, from sabotaging the enemy's docking towers, detaching their void grapnels and fusion drill heads, to counterattacking Word Bearers moving across the outer hull. Using void harnesses to bolster the in-built capabilities of Legiones Astartes power armour, the squads moved rapidly towards their targets, each step a Legionary took under the low gravity conditions powering him forwards tens of metres, each parabolic burn of his harnesses' thrusters carrying him high over the deep valleys and towering hills of the ship's exterior architecture. The Legionaries were dwarfed by the sheer scale of the flagship's form, a city-scaled landscape of iron and ceramite, the wounded surface of Calth visible as it rose above the port side attitude thrusters in a spectacle that under more conventional circumstances would have taken the breath away. But on this day of betrayal, all that the counter-boarding squads cared for was the survival of their flagship.The Ultramarines soon encountered their hated foe, for there was little possibility of a stealthy approach to battle in such conditions. The cobalt-armoured Ulttamarines had been observed as they traversed the slate-grey hull and soon mass reactive bolts were scything through the void on bright contrails. The Space Marines had been created for this mode of warfare and were equipped to the very highest standards, and so the battle was relentless and bitter. Bolt rounds which penetrated battle plate caused partial armour decompression before automated inner seals contained pressure loss, and even with a limb exposed to the vacuum, they could fight on for extended periods so long as oxygen reserves were not compromised. Soundless battle erupted across the hull, each Legionary able to hear only his own thunderous breathing and the urgent orders barked across the vox-net. When combatants clashed in melee, impacts sent victims rumbling away through the debris-strewn void, often trailing a cometary tail of blood globules and vented gas. Yet Chapter Master Empion's primary objective was not the destruction of the enemy boarding squads, as desirable a secondary mission as that clearly was to the Ultramarines. Three Legionaries in each squad carried a melta charge to sabotage the Word Bearers' heavy breaching equipment, and as the battle-brothers of both sides fought and bled, many of these specialists set to destroying the great fusion heads and boarding gantries with well-placed charges, many giving their lives in the process.At one such objective, the tide of battle had turned inexorably against Squad Six, led by Sergeant Thiel. The Word Bearers had counter-attacked in far greater numbers than anticipated and the surviving Ultramarines were resigned to extracting what vengeance they could before being overwhelmed and slaughtered. But such a fate was not to be and instead something that many later named a miracle occurred. From the void came a demi-god clad in armour of cobalt and pearl, an inaudible roar distorting his usually calm visage. It was Roboute Guilliman, Primarch of the Ultramarines, and he was as a raw force of nature, his usual measured demeanour driven out by cold fury as he slew Word Bearers by the dozen. As incredible as the fact of the Primarch surviving the destruction of the bridge was, what would mystify future generations of gene-smiths was the fact that the Emperor's son wore no battle helm. He had fought for ten hours in a vacuum without any apparent oxygen supply, a feat that even the preternatural physiology of the Primarchs could not entirely account for. Many accept that the body of a Primarch could withstand the effects of ebullism, hypocapnia, pressure-driven body mass expansion and extremes of temperature for extended periods. That a Primarch's body could continue to function without oxygen, however, is beyond any known capability of even these most awe inspiring of trans-human super-beings.The Primarch's intervention saved the lives of dozens of his sons. It fell to them though to persuade their gene-father to cease his furious battle on the flagship's outer hull and to rejoin his warriors within. The Primarch had fought for ten hours without respite, his sons believing him lost. Now he was returned to them, and for the first time since the beginning of the Battle of Calth, defeat and extinction did not seem inevitable.