The Mezoan Campaign was a battle of the Horus Heresy.
Shortly after his capture of Port Maw in the Battle of the Coronid Deeps, Warmaster Horus became determined to destroy the defiant Forge World of Mezoa. Two previous attempts had earlier been defeated by the loyalist Magos of the planet. Horus wished not only to make an example of Mezoa, he also sought to secure his rear as part of the push to Terra. Ultimately, it fell to the Iron Warriors 114th Grand Battalion of Nârik Dreygur and the 78th Chapter of the Alpha Legion under Autilon Skorr to undertake the operation. Resentment against the Alpha Legion by the Iron Warriors due to their behavior during the Siege of Epsilon-Stranivar IX hindered traitor cooperation and saw them compete over resources and prestige.
A few short days before the traitor assault began, the Salamanders Cruiser Ebon Drake arrived at Mezoa. The ship carried the Disciples of the Flames, a mixed loyalist force commanded by Cassian Dracos. The first phase of the battle saw traitor and loyalist forces engage in a massive 9 hour void duel that resulted in heavy losses for both sides. Despite the Traitor tactical superiority, they were hampered by the fractured nature of their own force. In the end however the traitors emerged victorious, and began full-scale invasion of the world. At this same time, the Disciples of the Flames under Dracos met with the triumvirate norn-regents of Mezoa, working out the details of an alliance.
The first wave of the traitor invasion consisted of an aerial landing by the Iron Warriors. Mezoa sent up vast projectiles of bedrock and magma from its furnaces, acting as air defenses which took a toll on the traitors. However they could not maintain this as a constant bombardment, allowing the Iron Warriors to launch a second wave that secured a foothold. The Iron Warriors were then informed by their Alpha Legion allies that they would be met with fleet fire if they attempted any retreat, leaving the only way forward. A vicious loyalist counterattack was met by the Iron Warriors, who fought with a ferocity resembling the World Eaters. Nârik Dreygur himself slew the Imperial Fists Champion Valtus Moran, tearing apart his corpse and causing a loyalist retreat. Thanks to the Iron Warriors sacrifice, the traitors had secured a landing zone.
Using landed Stormbirds Void Shield generators to create a canopy of protection from loyalist artillery, the traitors unloaded heavy equipment onto Mezoa. Besides Iron Warriors and Alpha Legion forces, the invasion force also included Space Marines of more obscure origin, including traitorous Iron Hands. The Traitors then used their advantage in numbers to make a major drive on Mezoa's Forges, and Autilon Skorr drove his massed Auxilia into a breach while the Iron Warriors were committed wherever the enemy was strongest. For their part, the loyalists used Battle Automata and Solar Auxilia to hold the line while the Disciples of the Flames conducted spoiling raids. But the traitors kept advancing, forcing the Tech-Priests of Mezoa to collapse entire regions of the planet's crust to bury enemy lines and hinder progress. Elsewhere, they used suicidal charges of Tech-Thralls to draw the enemy into the open before obliterating both sides with blasts of magma. For the next 9 days, Skorr led a bloody siege of Mezoa. However Skorr was not content to wage an extended siege, but rather the ambitious commander wanted to join the Siege of Terra to join in the glory. As a result, he rushed his attack.
Thus despite the protests of Dreygur the remains of the Iron Warriors 114th Grand Battalion were committed to a massive frontal assault on the defenses of Tertial-05, one of the key loyalist defense bastions around the main Forge spire. Despite heavy casualties, the Iron Warriors heavy infantry maintained a steady advance until a massed loyalist assault ground it to a halt. Despite not being able to survive without Alpha Legion reinforcements, no aid came to the Iron Warriors as XXth Legion Headhunter teams under Skorr himself were already waging their own attack on the forge spire. Battling Battle Automata and 50 Disciples and slaying one of the 3 Norn-Regents. With the death of one of their triads, chaos erupted across Mezoa as war robots went haywire and Skitarii and Thallax cohorts found themselves leaderless. It was at this moment that Dracos entered the fray, leading a charge of Disciples and 1,000 Battle-Automata on the Iron Warriors lines. Dracos reminded Dreygur that the path of honor remained, reminding the commander of the treachery his Alpha Legion allies had inflicted upon him. Convinced to defect, Dreygur's forces were added to Dracos' unstoppable wave. Skorr's remaining forces were overwhelmned and forced into a retreat all the way to his landing zone, and even this soon fell. The traitors fled Mezoa in disgrace.
In the aftermath of the battle, Skorr was badly wounded, having lost an arm to Xiaphas Jurr. Meanwhile, Dreygur and his Iron Warriors joined the Disciples of the Flames; Dreygur became a close confidant of Dracos. While little saw significance in the victory, it indeed slowed Horus' advance on Terra.
- 1: The Horus Heresy Book Six - Retribution, pgs. 38–44
- 2: The Horus Heresy Book Four - Conquest, pg. 19