Fluff time!
Battlefleet Urdesh has at its heart the forge world of Urdesh. Urdesh provided the bulk of the munitions required for the Sabbat Worlds Crusade to retake the Sabbat Worlds from Chaos. Briefly lost to Chaos itself, the forge world was swiftly retaken and continued supplying a steady stream of munitions to the Crusade effort.
Fed by this steady stream of munitions, Battlefleet Urdesh adapted to make heavy use of torpedos - ships were refit to carry torpedo systems, and launch bays to service torpedo bombers.
These tactics proved especially lethal at the battle of Aster's Reach, where the chaos grand cruiser Archon's Fist was pursued by the Dauntless class cruisers Sabbat's Fury and Blue Thunder for ten hours through the gas clouds, and eventually destroyed by repeated torpedo bombardment.
In another incident, the Chaos Battleship Crooked Rapier was lost in orbit of Denit VII after a surprise attack by torpedo bombers from the Dictator class cruiser Silver Glint and its Firedagger class escorts. The Crooked Rapier suffered crippling damage to its engine room and, unable to maintain orbit, it burned up in the swirling atmosphere of the gas giant.
The Chaos forces in the sector were quick to make use of their own advantages - Chaos ships would use their speed to appear seemingly out of nowhere, destroy lone capital ships, and vanish back into the void. Not even escorted cruisers were safe - the Tyrant class cruiser Dorn's Solace was lost during convoy duty when its squadron of Viper class escorts were drawn off by cunning Chaos tactics - a pair of Chaos frigates appeared as though to attack the convoy, and quickly retreated when the defenders made themselves known. Though the Viper squadron pursued and successfully destroyed the intruders, when they returned, Dorn's Solace and the convoy were no more than an expanding cloud of flaming debris, and an unidentified Carnage class cruiser slinking away.
To compound the problem, in spite of Imperial numerical superiority, Chaos intelligence always seemed to be two steps ahead of the Imperials, and for a time it seemed that any capital in the entire fleet left alone for more than a few hours would be ambushed and destroyed. Fleet HQ suspected a spy, but the incidents continued even after the entire fleet operations staff were executed and replaced, and no spy was ever found.
With no end to the incidents in sight, it was decreed no capital ship should ever be without an escort. Even after the Crusade, this decree was never repealed. Fleet HQ also petitioned the Forge World of Urdesh to refit more of their ships to battlecruiser status - it was hoped that if more ships that were deadlier than they looked, more raids could be foiled.
These tactics worked, although at cost - with the necessity of close escort, the Imperial fleet could be in fewer places at once, and raids against civilian targets intensified. Nevertheless, with Imperial losses were reduced, and the Crusade went on to acheive ultimate victory.
In the aftermath of the Crusade the Chaos fleet was scattered. Nevertheless, the raids continued, and it was found that Imperial vessels - even the newly refit battlecruisers - just did not have the speed required to bring the elusive Chaos forces to battle. Those lighter vessels that could catch the Chaos ships inevitably found themselves horribly outgunned. What was needed was a new type of vessel, one that could keep pace with the chaos vessels and obliterate them in a fight, and thus and that could operate free from the neccessity of an escort. Fleet HQ once more pettitioned the tech priests of Urdesh for help.
Thus was the Repulse Class Heavy Battlecruiser born. The first of the class, named Repulse after its intended function on the Chaos raiders, was converted from one of the fleet's successful Retribution class battleships - Bulkheads and shield generators were stripped out to make way for enlarged engine spaces, whilst heavy armoured turrets were stripped out to lighten the ship, reducing its dorsal lance battery to 2 and leaving the ship worryingly undefended against ordnance. It was also decided to strip out 1/3 of the ship's macrocannon batteries, replacing their heavy, armoured casemates and internal macrocannon magazines and replace them with launch bays - which were significantly lighter. Torpedoes for the Torpedo Bombers, though smaller than the vessel's main armament, were stored in the forward torpedo magazine, and a rail system was installed for delivery.
The class performed excellently in service and Fleet HQ soon ordered three more.
In M41.860 Commodore Ryken, captain of the Repulse suffered political embarrassment when, after attempting to engage the Chaos battleship Bone Render, Repulse had to spend six months in dock undergoing repairs. Though excellent against cruisers, the heavy battlecruiser concept couldn't stand up to a real battleship. Commodore Ryken swore both revenge, and to never make the same mistake again. Fourteen years later, Repulse once more found herself confronted with Bone Render. Rather than engage, Repulse shadowed the Chaos battleship for three weeks and even through six warp transitions until a task force could be sent to assist and foul vessel was finally brought to justice.
