Traitor Titan Legions - Horus Heresy: Battle of Isstvan III (005-006.M31) - The Warmaster Horus revealed the terrible scale of his treacherous ambition and the destruction wrought by the battles of the Horus Heresy to come when he engineered the betrayal of the Loyalist Space Marines at Isstvan III, purging his own and three other Space Marine Legions of those Astartes who could not be relied upon to swear to his cause to overthrow the Emperor of Mankind. The slaughter is made all the more terrible by the Warmaster's alliance with the Legio Mortis ("Death's Heads"), the Legio Audax ("Ember Wolves") and the Legio Vulpa ("Death Stalkers"), whose god-engines are impervious to the Life-Eater virus unleashed against the betrayed Astartes, and which stride through the firestorms that result from the virus-bombing of that world as if they were the towering and vengeful giants of ancient legend.Schism of Mars (005-006.M31) - Kelbor-Hal, the fabricator-general of Mars, declared the secession of Mars from the Imperium and the ancient Mechanicum's rejection of the Emperor of Mankind as the Omnissiah in a planet-wide betrayal co-aligned to that of the Warmaster Horus at Isstvan III. Led by the Legio Mortis ("Death's Heads"), the Traitor Mechanicum, later called the "Dark Mechanicum," scoured the surface of the Red Planet of those still loyal to Terra, plunging the first and most important Forge World of Mankind into a bitter civil war that saw the extinction of several ancient Collegia Titanica Legios and dozens of Knight houses. The last to fall were the Loyalist hold-outs of the Legio Tempestus ("Stormlords"), who, perhaps mercifully, died unaware that the bulk of their Legio off-planet had renounced their oaths to the Emperor and declared for the Warmaster.Drop Site Massacre of Isstvan V (006.M31) - After word of his actions at Isstvan III had reached Terra thanks to the escape of a handful of Loyalists aboard the captured Death Guard frigate Eisenstein, the Warmaster Horus conceded the advantage of surprise and was unable to launch an immediate assault on Terra. Instead, he lured three Space Marine Legions -- the Iron Hands, Raven Guard and Salamanders -- into a trap at the nearby world of Isstvan V, slaughtering hundreds of thousands of Astartes in less than three solar hours of fratricide. Though primarily a matter of honour to be settled between the Legiones Astartes, both sides are bolstered by the presence of allied Titan Legions, the Loyalists by the Legio Atarus ("Firebrands") and the Traitors by the well-blooded Legio Mortis ("Death's Heads"). By the slaughter's end, the Death's Heads are victorious, the burning wrecks of the Firebrands' god-engines now added to the vast fields of Space Marine corpses.First Battle of Paramar (006.M31) - Participating in an operation by the primarch of the Alpha Legion, Alpharius, in the immediate aftermath of the Isstvan battles intended to capture the Mechanicum provender world of Paramar V, the Traitor Alpha Legion and the allied Legio Fureans ("Tiger Eyes") god-engines anticipated a quick capture of the Paramar Nexus. After pressing towards their final objective, however, the Traitors were surprised to encounter not just an entire Grand Company of Iron Warriors still staunchly loyal to the Emperor, but a large force of the Loyalist Legio Gryphonicus ("War Griffons"). The First Battle of Paramar proved a victory for the Traitors, albeit a more costly one that they had anticipated, but it is just the first in a series of battles fought during and after the Horus Heresy for control of this strategically vital star system.Battle of Calth (007.M31) - Their Space Marine Legions united in dark purpose, the Primarchs Lorgar of the Word Bearers and Angron of the World Eaters voyaged the length of the galaxy and fell upon the Five Hundred Worlds of the Realm of Ultramar. Unaware of the outbreak of a galactic civil war, the Ultramarines Legion welcomed the Word Bearers as brothers, only to be betrayed at the moment the hand of friendship was extended. Calth was made by Lorgar's forces into a dark offering to the Chaos Gods that he and his Legion had long served since the Pilgrimage of Lorgar forty standard years before. The galaxy was split asunder by the massive Warp Storm known as the "Ruinstorm" that the Word Bearers used the bloodshed on Calth to summon. During the conflict on Calth, the Legio Suturvora ("Fire Masters") betrayed its oath to Terra, while the Legio Praesagius ("True Messengers") and Legio Oberon ("Death Bolts II") remained staunchly loyal. Mustered together at the moment of treachery, the Fire Masters' god-engines opened fire on a heavy conveyor within which dozens of True Messengers god-engines were embarked, a bitter battle erupting to free the surviving god-engines from the downed wreckage of the spacecraft. Later still, a force of True Messengers martyred themselves to their Fire Masters foes during the battle for the Calthian city of Ithraca, fighting to the last against the unleashed forces of Chaos rather than retreating from the city and falling prey to the enemy's orbital artillery.008.M31 Ruin of Maerdan - In the third standard year of the Horus Heresy, the Imperial colony world of Maerdan on the edge of the Segmentum Solar became a savagely contested frontline between the forces of Loyalists and Traitors. Maerdan's cities became flaming ruins and its once verdant plains were turned to barren wastes, crushed and scoured by the fury of the battle as the Loyalist Titans of the Legio Gryphonicus ("War Griffons"), Legio Metalica ("Iron Skulls") and the Legio Destructor ("Beasts of Stell") clash with the Traitors of the Legio Mortis ("Death's Heads"), Legio Argentum ("Dread Lances") and the Legio Vulturum ("Gore Crows"). The Ruin of Maerdan, as the campaign comes to be known to later Imperial historitors, is one of the largest Titan battles of the early Horus Heresy, when more than 200 Titans took to the field against each other. Heavy losses were incurred by both sides and as the war moved on, it left a shattered world behind, with neither Loyalists nor Traitors able to claim a true victory. The flames of hatred between the War Griffons and the Death's Heads, once staunch allies, burn bright after Maerdan and their enmity would lead them to seek each other out time and again on the fields of battle in the dark years that followed their first clash.Battle of Molech (009.M31) - The Traitor hosts marching towards Terra under the command of the Warmaster Horus reached the Knight World of Molech, the scene of a great Imperial victory during the Great Crusade and heavily garrisoned for reasons known originally only to the Emperor Himself and later discovered by the Warmaster -- that the world hosted an ancient Warp Gate that allowed entry into the Realm of Chaos for discourse with the Ruinous Powers themselves. This was knowledge that the Emperor had psychically erased from the minds of His primarchs after their participation in the original battle that had brought Molech into Imperial Compliance. The Traitors committed an enormous invasion force, including the bulk of the Sons of Horus and Death Guard Legions, and Titans of the Legio Mortis ("Death's Heads"), Legio Vulcanum II ("Lords of Ruin"), Legio Interfector ("Murder Lords"), and Legio Vulpa ("Death Stalkers"), which were opposed by elements of the Ultramarines and Blood Angels Legions, Titans of the Legio Gryphonicus ("War Griffons"), Legio Fortidus ("Dauntless"), and Legio Crucius ("Warmongers"), and hosts of Knights from almost a dozen houses, with both sides bolstered by hundreds of thousands of allied mortal troops. At the height of the battle, House Devine revealed that its true allegiance was to the Warmaster, its treachery unveiled at a pivotal moment, heralding a slaughter from which only a handful of Loyalists escaped. The Warmaster went on to claim the world and enter the Warp Gate, from which he emerged fully empowered as the chosen champion of the four major Chaos Gods in the guise of Chaos Undivided.Battle of Tallarn (010-012.M31) - Perturabo, primarch of the Iron Warriors Legion, launched an all-out planetary invasion of the verdant Imperial staging world of Tallarn, his Legion bolstered by dozens of allied Traitor Imperial Army regiments, the Titans of the Legio Krytos ("God Breakers") and the Knights of House Caesarean. Both the Loyalist and the Traitor high commands are taken by surprise by Perturabo's actions and the invasion quickly escalated after he ordered an Exterminatus action upon the world, scouring its surface of living matter by way of a voracious Life-Eater viral barrage. While the population is all but wiped out, many defenders survived thanks to the existence of extensive subterranean shelters. The ensuing campaign is fought between vast formations of Imperial Army and Solar Auxilia tanks, the Titans of the Legio Gryphonicus ("War Griffons"), the Knights of House Megron, the indentured automata of the house army of the Rogue Trader Sangrea as well as armoured forces of the Iron Hands, Imperial Fists, White Scars and Ultramarines Legions. No infantry can survive in the poisoned wastes of Tallarn, and the war quickly drew in other forces from across the region. The Battle of Tallarn is now considered the largest armoured engagement in the know history of Humanity, and while counted as a victory for the Loyalists, millions of warriors and war machines on both sides were left scattered across the lifeless, deadly surface of the planet.Cataclysm of Iron (010.M31) - Across the border sectors of the Segmentum Tempestus and Segmentum Pacificus are located numerous lesser Forge Worlds known collectively as the "Belt of Iron." Since the sundering of the Imperium, many declared for the Traitor cause at the engineering of the fabricator-general of Mars, while others have remained loyal to Terra or sought to remain as aloof as possible from the conflict. Strife and tentative conflict between these once-aligned worlds erupted into full-scale war in 010.M31, which pitched the Mars-aligned Dark Mechanicum Forge Worlds of Incunabula, Urdesh, Valia-Maximal and Kalibrax against the forces of the Loyalist Forge Worlds of Graia, Arl'yeth and Atar-Median, while Arachnis and Jerulas Station both fall into civil war. The resulting conflict, that later became known as the "Cataclysm of Iron," saw the forge lords, their armies and allied Titan Legions and Knight houses turn on each other in protracted warfare, while scores of Human-inhabited worlds in the region suffered as they became the battlegrounds upon which they fought.Battle of Nyrcon (010.M31) - Under the direction of Rogal Dorn, a Loyalist battlegroup led by elements of the Salamanders Legion, and supported by thirty regiments of Solar Auxilia and a strike force of Legio Astorum ("Warp Runners") Titans, engaged the Emperor's Children millennial assigned to garrison the Beta-Garmon System on the edge of the Segmentum Solar. The Emperor's Children contingent is surprised at Nyrcon City on the world of Beta-Garmon II and after a hard-fought battle, the Emperor's Children forces, along with their Legio Mortis ("Death's Heads") allies at Beta-Garmon III, are driven out, placing the system into Loyalist hands and triggering within the standard year the first in a escalating series of Traitor counter-attacks.Second Battle of Paramar (011.M31) - Paramar V had fallen to the Warmaster Horus' hosts in the opening moves of the Horus Heresy. In response, a mixed force of Loyalists launched an assault against the strategically vital supply nexus with the intent of denying it to the Traitors' war effort. The Traitors numbered a substantial combined forces of Legio Fureans ("Tiger Eyes") and Legio Mortis ("Death's Heads") god-engines that were in the system re-arming and re-supplying after several standard years of intensive campaigning, and a large presence of Sons of Horus and Word Bearers Heretic Astartes. The Loyalists committed a large Titan force drawn from the Legio Atarus ("Firebrands"), Legio Ignatum ("Fire Wasps") and Legio Solaria ("Imperial Hunters"), with ground assault units of the Blood Angels and White Scars Legions. The Loyalists conducted a series of diversionary attacks across the Paramar System in order to draw Traitor forces away from their true target, before they conducted a full-scale planetstrike against Paramar V's primary spaceport, capturing it intact and then pressing outwards to begin the destruction of the mass-provender silos that sprawled across the plateau beyond. It was soon revealed, however, that the Traitors had seen through the Loyalist ploy and prepared a huge counter-attack force. Though the Loyalists inflicted heavy damage on the provender silos before the counter-attack hit home, the vast majority of the Loyalist invasion force was surrounded and destroyed without mercy.Battle of Ice World Tralsak (011.M31) - The shallow, frozen oceans of the Ice World of Tralsak were flash-boiled to steam by the fury of god-engine war as the Loyalist Titans of the Legio Atarus ("Firebrands") and the Legio Agravides ("Battle Scourges") clashed with the Traitors of the Legio Magna ("Flaming Skulls") and Legio Victorum ("Foe Slayers") as Loyalist Shattered Legions and Traitor World Eaters Astartes fought across the disintegrating landscape of ice floes. Ultimately, outright victory eluded both sides, and each used the cover of the world-enveloping fog thrown up by the destruction to extricate their forces and redeploy them elsewhere. Nevertheless, numerous supporting Knights of both sides remained to fight a war in which neither side conceded defeat.Tarren Suppression (011.M31) - The world of Tarren IV declared for the cause of Horus, its rulers having fallen for the honeyed words of the Warmaster's emmisaries. A Loyalist Retribution Fleet moved quickly to crush the treachery, and its forces occupied Tarren IV's capital city of Brandstat and stamped out anti-Imperial sentiment in a brutal mirror of the Traitors' so-called "Dark Compliance" of conquered worlds. The key to the suppression were the Titans of the Legio Defensor ("Nova Guard"), who bestrode the surface of Tarren IV, the mournful dirge of their war sirens demanding submission to the Emperor and making examples of those cities that refused to do so by blasting them into flaming ruins. As if to mock the expected order of things still further, later on, it was an allied force of the Emperor's Children Legion, the Legio Cybernetica and the Legio Mortis ("Death's Heads") that conducted a heavy planetary landing and "liberated" the world from the Loyalists during the campaign known to the people of Tarren IV as the "Relief of Brandstat."Balthor Sigma Intervention (012.M31) - A blood-maddened pursuit force of Traitor World Eaters Astartes engaged in hunter-killer operations against defeated Loyalist forces was counter-attacked by a Legio Osedax ("Cockatrices") demi-legio at the world of Balthor Sigma. The World Eaters are supported by numerous super-heavy tanks and therefore are able to hold the Cockatrices Titans at bay until, in quick succession, the Traitor tanks are engaged from an unexpected quarter and destroyed in short order. Entirely unheralded, a force of xenos Titan-analogues, later determined to belong to the Eldar, intervened to devastating effect. It was only when Traitor-aligned Titans of the Legio Fureans ("Tiger Eyes") fought their way through to reinforce the Traitors' lines that total defeat for the Warmaster's servants was averted. At the conclusion of the battle, the xenos Titans disengaged and vanished into the ash-shrouded depths of Balthor Sigma's equatorial magma wastes.Founding of the Adeptus Mechanicus (012.M31) - A long-running deadlock within the Imperial Council on Terra was finally broken, which facilitated the formal establishment of the Imperial Adeptus Mechanicus, now separate and distinct from the ancient Mechanicum of Mars which had fallen to the service of Horus. Fabricator-General Zagreus Kane is appointed as the Adeptus Mechanicus' new representative to the council and is named a High Lord of Terra. Final judgment is enacted upon the captive followers of the Dark Mechanicum's Fabricator-General Kelbor-Hal, who are executed in their thousands by Imperial authorities. Immediately after, the union of the the Loyalist Titan Orders, under the umbrella of the reformed Adeptus Titanicus (Collegia Titanica), is formed by the grand masters of several major loyal Titan Legions, although the future of other loyal former Mechanicum divisions such as the Legio Cybernetica" and the Taghmata remained unclear.Imperial Muster at Beta-Garmon (012.M31) - For reasons only understood later in the war, the Ruinstorm that had been summoned by the Primarch Lorgar's servants among the Word Bearers Legion several standard years before during the Battle of Calth began to abate across vast swathes of the galaxy, reopening astropathic communications and transport routes to Terra. Loyalist high commanders were able to utilise the potent astropathic relay located in the Beta-Garmon System to contact and recall formerly lost or stranded Imperial forces from across the wartorn reaches of the northern Imperium. The Imperial muster at Beta-Garmon raoidly escalated into the largest gathering of Loyalist might since the early years of the civil war, a concentration of force so mighty that the Traitors were forced to respond in kind. The resulting clash of arms, the Battle of Beta-Garmon, would also become known as the "Great Slaughter," a theatre of war of such scale that it would encompass the Titandeath and the Sea of Fire campaigns, each major battles in their own right. By its end, both sides were so badly bled that their only remaining option for ultimate victory was to force a final, decisive confrontation. As history relates, that final battle would be fought at Terra, but only after the weighty butcher's bill of the Great Slaughter was paid in full...Scouring of the Ollanz Cluster (012.M31) - The Warmaster Horus' advance on Terra eventually reached the Ollanz Cluster, where he encountered Loyalist reinforcements bound for Beta-Garmon rearming and refuelling at the Borman System. The Loyalist fleet rapidly redeployed to oppose the Traitors' advance and to protect the valuable resource worlds of the stellar cluster. The turning point  of the brief but intense conflict came at Borman IV, when a Legio Astorum ("Warp Runners") demi-legio launched a daring offensive towards the world's beleaguered capital city. The crucial Titan battle took place in the volcanic Yrevendi Desert immediately to the north of the capital, as Princeps Senioris Varr Harax led a surprise thrust through rough terrain at a weak point in the Legio Fureans ("Tiger Eyes") lines, accompanied by Astartes ground forces drawn from the Iron Hands Legion. The Tiger Eyes, having been drawn away by a bold diversionary attack, were annihilated by the Warp Runners, who were able to break through the Traitor Emperor's Children Legion's lines and reinforce the beleaguered planetary capital. Ultimately, Borman IV and the entire Ollanz Cluster were delivered from the Traitors' possession, which allowed the Loyalist forces to be redeployed to their original destination at Beta-Garmon.