Word Bearers - Furious Abyss and the Battle of Calth: As the Horus Heresy unfolded, the Word Bearers' unique and monstrous Battleship Furious Abyss, set off for the Realm of Ultramar in the Eastern Fringes of the galaxy after its secret construction by the Dark Mechanicus was completed on the Jovian moon of Thule. The Word Bearers intended to gain vengeance upon their old rivals the Ultramarines and pin their forces down in Ultramar while the main forces of the Traitor Legions advanced on Terra under the command of Horus.The Furious Abyss ' first kill was the Ultramarines starship Fist of Macragge as it was heading for repairs to the Vangelis spaceport. The Astropaths of the Fist of Macragge managed to send out a telepathic warning through the Warp, ultimately received as a very powerful psychic scream at the Vangelis spaceport.Seeing visions of Macragge and the impending terror brought by the Traitor Marines of the Word Bearers Legion, Captain Cestus, the Fleet Commander of the Ultramarines Legion, quickly gathered a force of starships at the main Vangelis hub of Coralis. He managed to find seven Loyalist starships: the Wrathful (which Cestus commandeered as his flagship) and her Escorts, the Fearless, the Ferox, the Ferocious, the Fireblade, and the Thousand Sons Legion's starship Waning Moon.Captain Vorlov of the starship Boundless also requested to join Captain Cestus, and was accepted. The Loyalist fleet pursued the Furious Abyss and assaulted it, suffering heavy casualties as all of the Loyalist starships but the Wrathful and Fireblade were destroyed. The battle ended only when the Furious Abyss entered the Warp to continue its journey to Ultramar.The Wrathful and Fireblade made the transition as well, but the Furious Abyss deployed a psionic mine which disturbed the Warp, causing the Fireblade's Gellar Field to fail, and the ship was torn apart by the daemonic forces of the Empyrean.When the Word Bearers of the Furious Abyss finally launched their attack against the Ultramarines' dominion of Ultramar, the strike against the world of Calth was led by Lorgar's greatest champion, the former Master of the Faith, First Captain Kor Phaeron. He swore to utterly destroy the Ultramarines' homeworld of Macragge in vengeance for what the Ultramarines had done to Khur, and was very nearly successful. From his personal Battle Barge, now renamed Infidus Imperator ("False Emperor"), Kor Phaeron directed a full-scale invasion of the Calth System.Calth's three sister planets were all destroyed, massive geo-nuclear strikes ripping them apart at the core. The system's once gentle sun was laced with deadly heavy metals and other unstable substances that increased the star's radiation output tenfold; within a century after the Heresy's end, the final elements of Calth's atmosphere were burned off and the world was left an airless void, its populace forced to live in gigantic underground caverns which were transformed into massive subterranean hive cities.The Battle of Calth itself was devastating and horrific. The Ultramarines were shocked by the millions of Chaos Cultists the Word Bearers used as human shields and disgusted by the hordes of daemons they unleashed from the Warp. The Word Bearers, in turn, had underestimated the tenacity and resolve of their Loyalist foes. The Loyalist 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 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.At the same time, the Word Bearers had used the battle taking place on Calth to 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 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 XIIIth 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.In the end, Kor Phaeron had been defeated when Ultramarines reinforcements from Macragge translated from the Warp above Calth and drove the Word Bearers Astartes from the surface of Calth. Kor Phaeron retreated all the way to the Maelstrom, a turbulent Warp rift much like the far larger Eye of Terror where the Immaterium of Chaos seeped through into the material realm of the universe.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. Guilliman immediately gathered reinforcements in the wake of the Calth Atrocity and swore to hunt down his brother Lorgar and make the Word Bearers pay for what the treachery they had unleashed upon Ultramar and its people.