The Siege of Epsilon-Stranivar IX was the first major setback encountered by the Traitor Legions on their long march to Terra during the Horus Heresy in 007.M31. A former lynchpin of the Great Crusade, "Fortress Stranivar", as is was commonly called, became a rallying point for those Imperial formations that had successfully escaped the onslaught of the first wars of the Horus Heresy such as the Drop Site Massacre and the Manachean War. Despite facing overwhelming odds, the unity of purpose of the surviving Loyalists was enough to repel an ill-conducted siege by the Traitor forces which would set back Horus' campaign by almost a solar year. It was a proud Imperial victory prominently displayed in several Loyalist Legions' battle honours and an abject failure for one of the enemy's rising commanders, Autilon Skorr of the Alpha Legion, which would eventually force him into conducting the disastruous Mezoan Campaign in a vain attempt to rise against in the Warmaster Horus' favour.
Due to its strategic location at the nexus point of several stable Warp routes linking the Coronid Deeps to its neighbouring sectors and even the Segmentum Solar, the world of Epsilon-Stranivar IX had become a beachhead of the ever-expanding Great Crusade as it ventured further into the Segmentum Obscurus. By the end of the Great Crusade in the early 31st Millennium, Epsilon-Stranivar IX had become a Fortress World, its barracks and mustering grounds harbouring countless regiments of the Excertus Imperialis ready to join the Crusade. With the beginning of the Horus Heresy, "Fortress Stranivar", as it was widely known, could again fulfill its intended purpose: a secure beachhead at the Imperial frontier with the Coronid Deeps which had since been conquered by the Traitor Legions and now formed the heartlands of their secessionist empire. Fully aware of the fact that while the Loyalists held Stranivar his rebellion was exposed to a swift counterattack by Loyalist forces, Horus made the conquest of this world one of his early priorities and tasked none other than the grim Primarch of the Death Guard, Mortarion, with its capture.
Indeed, Fortress Stranivar had been designed and was garrisoned by a detachment of the Imperial Fists, the sons of the great Primarch Rogal Dorn and whose expertise in siegecraft was unparalleled, save perhaps for the dour warriors of the IV Legion, the Iron Warriors. As the first battles of the Heresy unfolded, Stranivar became a safe haven for those Loyalist forces that had managed to escape alive from the deadly trap of the Drop Site Massacre or other conflicts. Therefore, the Loyalist Legiones Astartes presence, which had only numbered a few hundred so far, swelled to considerable strength, reinforced by survivors of the three Shattered Legions: the Iron Hands, the Raven Guard and the Salamanders. Escaping the clutches of the enemy, even an errant strikeforce of the Blood Angels -- whose Legion had been deployed far to the galactic south -- found its way to Stranivar. Horrified by the news of treason brought by the survivors of Istvaan V, the Imperial Fists and their allies swore solemn oaths to make the Traitors pay and fight to their last breath.
While the Loyalists fought shoulder-to-shoulder as true brothers, the Warmaster's armies showed a remarkable lack of unity. First of all, Mortarion focused his attention solely on his own Legion, prosecuting the war according to the tenets of the XIV Legion as they had proven highly efficient in the past. At no time did he seek the counsel or indeed the involvement of his allies, who he let devise their own strategies and fight when and where they chose. Those allies included the 78th Chapter of the Alpha Legion and the 114th Grand Battalion of the Iron Warriors, whose knowledge of siege warfare could indeed have turned the fight in the Traitors' favour -- if they had not been squandered. Mortarion's lack of interest quickly became a cause of friction, as both the Death Guard and the Alpha Legion frequently let the 114th Grand Battalion attack the heavily fortified Loyalist positions unsupported. Inspired by the ruthless tactics employed by his Legion at the First Battle of Paramar, the Alpha Legion's commander, Autilon Skorr, used the sacrifice of the 114th Grand Battalion to his own profit, conducting surgical strikes on high-value targets and claiming all the glory for himself whilst the Iron Warriors bled and died unacknowledged and unnoticed.
At the Siege of Vharna, when the Loyalists defiantly vanquished their opponents, the Iron Warriors were abandoned by both the Death Guard and the Alpha Legion, the Grand Battalion left behind to act as a sacrificial rearguard whilst their allies escaped. The 114th Grand Battalion lost nearly 4,000 Astartes in that fight, leaving its highest-ranking officer with barely 2,000 Astartes under his command. Embittered by this turn of events, Nârik Dreygur swore that his fallen brothers would be avenged, sowing a seed of discord that would soon bear terrible fruit.
For the Loyalists, the defence of Epsilon-Stranivar IX was a much-needed victory, which was celebrated as such. News of this first success would embolden those elements of the Shattered Legions and the Loyalist defenders of the Segmentum Obscurus to go on fighting. The outcome of the siege was the first major setback of the war for Horus, who had to redeploy and allot new troops for a second attempt at taking Stranivar. Although this second attack less than a solar year later indeed captured world, the damage to the Traitors had already been dealt. Whilst due to his status as a Primarch, Mortarion was never blamed, both Autilon Skorr and Nârik Dreygur returned to the Warmaster in shame. Weakened by their failure, it would be these two commanders who would be charged with conducting the brutal Mezoan Campaign to finally bring the Loyalist Forge World of Mezoa to heel. On the dark volcanic soil of Mezoa the seed of discord sown at Epsilon-Stranivar IX would finally blossom in hideous form, as the 114th Grand Battalion finally earned its vengeance by turning its guns on the Alpha Legion when the insidious XX Legion betrayed them yet again.
- The Horus Heresy - Book Six: Retribution (Forge World Series) by Alan Bligh, pg. 34