Ultramarines - Battle of Calth: When the Warmaster Horus turned his back on the Imperium, swore his allegiance to the Ruinous Powers of Chaos, and began the great rebellion against the Emperor later named the Horus Heresy, his first act before making his break with the Emperor of Mankind open was to lure away as many Loyalist Legions from the defence of Terra as possible.As part of this ruse, Horus ordered Roboute Guilliman to lead an expeditionary fleet to the world of Calth in the Veridian System in the Realm of Ultramar to prepare for a campaign in the Eastern Fringes of the galaxy, where, Horus claimed, an Ork WAAAGH! was massing on the Ghaslakh Xeno-hold.Horus expected the Ultramarines to await the arrival of the Word Bearers who would join with the XIII Legion in prosecuting a campaign against the Ork menace. Unknown to Guilliman, the XVII Legion had four solar decades before turned Traitor in service to the Chaos Gods, and its primarch, Lorgar Aurelian, gleefully accepted Horus' orders to close the trap on his Legion's long-hated rivals.The Word Bearers' sudden attack at Calth decimated Guilliman's Legion fleet, and the Ultramarines' ground troops quickly found themselves impossibly outnumbered by their former allies as the infamous Battle of Calth erupted. The Word Bearers slew their Loyalist foes in droves in the early stages of their surprise attack and pushed them back over huge stretches of territory.The Traitors rejoiced at the terrible blows they were inflicting upon the Legion that had once aided the Emperor in humiliating them upon the world of Khur solar decades before the start of the Heresy when the Word Bearers had been taken to task for repeated violations of the atheistic philosophy known as the Imperial Truth.Unknown to them, Guilliman's flagship, which had survived the initial Word Bearers' attack on the Ultramarines fleet, effected emergency repairs and regrouped with the other surviving Ultramarine starships in space. Having taken stock of his remaining forces, Guilliman sent an immediate astropathic distress call to Macragge.The Loyalist Space Marines on Calth, Ultramarines all, had been forced into a fighting retreat, but soon occupied fortified positions. Many Ultramarines had been born on Calth, and proved more resolute than the Word Bearers anticipated. In space, Guilliman's vessels began hit-and-run attacks on their over-confident enemy. Guilliman assessed his ground troops' positions and broadcast clear, concise orders to each pocket of defence, coordinating them into a cohesive force. One Ultramarine force led by Captain Remus Ventanus led a breakout and retook Calth's Defence Laser silos, aiding the sorely-pressed Ultramarines fleet from the surface of Calth.Guilliman's depleted forces slowed the Word Bearers down long enough for the remainder of the Ultramarines Legion to arrive and rout the Traitor Marines from the system, though at a heavy cost. The Word Bearers turned Calth's own orbital defence platforms on the Veridian star, stripping away the outer layers of its photosphere and destabilising it, ultimately rendering the surface of Calth uninhabitable due to a massive increase in ionising radiation bursts from the world's sun.At the same time, the Word Bearers had used the battle taking place on Calth to fuel the Warp sorcery required summon a massive Warp storm called the "Ruinstorm" that was intended to cut off Ultramar from the rest of the galaxy and prevent the Ultramarines and other Loyalist forces from providing any reinforcements to Terra as Horus made his assault upon Humanity's homeworld.The eruption of the Ruinstorm cut off Calth from the main body of the Ultramarines Legion and left the Astartes of the XIII Legion trapped on Calth, locked in a brutal subterranean war with those Word Bearers units that had also been left behind when their Legion retreated from the Viridian System.Yet Roboute Guilliman and a large portion of his Legion had remained off-world as a result of the Word Bearers' devious assault upon the Ultramarines fleet. Bloodied but unbowed, the Ultramarines received the orders of Malcador the Sigillite, the Regent of Terra, while the Emperor was in disposed pursuing the secret Imperial Webway Project, and prepared to meet the needs of the Imperium's defence against the Traitor Legions as best they could.