Battle for the Abyss by Ben Counter begins with the construction of the gigantic battleship, the Furious Abyss, within the hollow center of Thule, a moon of Saturn. The Mechanicum construct the ship using the ancient technologies they preserve. Unbeknownst to the Emperor, the Mechanicum build the massive warship for the Word Bearer Traitor Legion. Those familiar with the Cylon basestars of Battlestar Galactica will recognize the Furious Abyss. Heavily armed and holding a contingent of fighters, the Furious Abyss is an intimidating force. Unlike the sleek basestars, the Furious Abyss resembles a giant battlestar with Chartres Cathedral sitting on top.
In the novel, we meet several Space Marine legions, each with their own specialty and genetic modification. The aforementioned Word Bearers are a Traitor Legion combining martial skill with a fanatical adherence to the Word of Lorgar, their Primarch. In the unfolding galaxy-spanning civil war, the Word Bearers resemble Oliver Cromwell at his most theocratic, fanatic, and tyrannical.
Members of the five legions meet up on Vangelis to resupply their ships. Everything proceeds apace, with the Space Marines prepping themselves for their future engagement, to meet at Calth “in preparation to launch a strike on an ork invasion force besieging the worlds of the neighboring Veridan.”
Following a psychic attack on Vangelis, Cestus, Brother-captain and fleet commander of the Ultramarines 7th Company, discovers that the Wrathful Abyss will strike the Ultramarines homeworld of Ultramar. Cestus commandeers the Wrathful, a ship of the legendary Saturnine Fleet. The Fleet has a history that predates the Empire of Man. Members from three other Space Marine Legions accompany Cestus. Skraal, Brother-captain of the World Eaters Legion, fights with a psychotic ferocity that frightens the other Space Marines. Brynngar, Captain of the Space Wolves Legion, with his lupine incisors and penchant for drinking, has serious misgivings about Mhotep, Brother-sergeant of the Thousand Sons. Mhotep raises Brynngar’s ire because the Thousand Sons, shunned at the Council of Nikea due to their psychic abilities, embody an irrational, unknown force. At this stage of Imperial history, people possessing psychic powers still pose a threat to the Emperor’s embrace of rationalism and reason. The Word Bearers broke their oath with the Imperium because their fanaticism and superstition met with censure from the Emperor.