Lion El'Jonson studied the glowing motes of data and folded his arms, his expression distant and thoughtful. Nemiel watched another ghostly smile play across the primarch's face as both forces arrayed themselves for battle, and fought down another twinge of unease. At that moment he would have given a great deal to know what Jonson saw in the grim picture that he did not.
As soon as the Dark Angels' battle group had arrived in the Gehinnon star system it had effectively split into two forces. Six of the group's sixteen ships were sleek, swift destroyers, which the primarch immediately ordered ahead of the main division with a trio of light cruisers to provide support. These scout squadrons quickly pulled ahead of the larger and slower cruisers, their long-range surveyors sweeping the void ahead of them and attempting to fix the size and disposition of the enemy fleet.
Now that the enemy was sighted, vox signals went back and forth between the two destroyer squadrons and the trio of light cruisers hanging back in their wake. As the rebel picket ships - no less than fifteen enemy destroyers, organised into three large squadrons - deployed into a standard crescent formation, Jonson's light cruisers flared their thrusters and moved up to form a battle line with the rest of the scouts.
Thousands of kilometres behind them, the main body of Jonson's battle group was altering formation as well. The Invincible Reason and the strike cruisers Amadis and Adzikel drew ahead of the two grand cruisers and two heavy cruisers that comprised the rest of the main force. At the same time, the armoured blast doors covering the three ships' prow hangar bays slid ponderously open and flight after flight of Stormbirds leapt like loosed arrows into the darkness. Within minutes, seven squadrons of the heavily-armed assault craft were speeding ahead of the formation, racing to join up with the distant scouts before the rebel destroyers reached extreme firing range.
With four minutes left to contact, the rebel pickets suddenly increased speed; perhaps the flotilla commander detected the oncoming Stormbirds, or gave in to his eagerness to open the engagement, but it was too little, too late. Jonson's Stormbirds were streaking through the scout squadron's firing line just as the enemy destroyers opened fire.
The rebel ships opened the engagement as Jonson expected they would, opening their bow tubes and launching a salvo of deadly torpedoes at the oncoming scouts. Thirty of the huge missiles - each one powerful enough to blow a destroyer-sized ship apart - sped towards the scouts in a wide arc that left the Imperial ships with no room to escape.
Surveyor arrays aboard the Stormbirds detected the launches at once, and the Astartes pilots spread out their formations as widely as possible to intercept the oncoming torpedoes. They swept through the volley of missiles in the space of a few seconds; lascannons spat bolts of searing light, spearing through the torpedoes' casings and detonating their huge fuel tanks. Massive explosions flickered angrily in the darkness in the Stormbirds' wake, spreading clouds of incandescent gas and debris that faded quickly in the airless void. Almost half of the torpedoes were destroyed; the rest sped onward towards their targets, too fast for the assault ships to alter course and come around for another pass. The Astartes held their course, already picking out targets among the oncoming picket ships.
The scout squadrons opened fire on the incoming missiles as soon as they came within range. Macro cannons and rapid-cycle megalasers filled the vacuum ahead of the small ships with a veritable wall of fire. Energy lances - massive beams of voltaic power - swept in burning arcs ahead of the light cruisers. More globes of flame bloomed along the path of the onrushing scouts, blending together into a seething field of vaporised metal and radioactive gas.
Five torpedoes slipped through the maelstrom. They crossed the remaining space to their targets in less than a second, flying into a second, smaller cloud of exploding shells as the destroyers' flak batteries opened fire. The servitor-crewed guns succeeded in destroying two of the remaining missiles.
Three torpedoes out of thirty struck home. One of the weapons smashed into the prow of the destroyer Audacious but failed to detonate; Hotspur and Stiletto, however, were not so fortunate. The torpedoes' plasma warheads tore the lightly-armoured destroyers apart, transforming them into expanding clouds of gas and debris in a single instant. Horus's rebels had claimed first blood.
The surviving ships passed through the remnant gases of the intercepted torpedoes, wreathing their void shields with streamers of plasma and temporarily fouling their auspex returns. Hungry for vengeance, their surveyor crews strained at their scopes, searching for engine telltales amid the storm of interference. Moments passed; points of heat swelled like stars in the radioactive haze. Ranges and vectors were calculated and relayed down to the torpedomen, who entered the data into their deadly charges. While the enemy pickets were still trying to reload their tubes, the scouts launched a torpedo salvo of their own.
By this time, the two formations were at extreme weapons' range, and the enemy pickets were faced with a dilemma: fire at the oncoming Stormbirds, the torpedo salvo or the scout squadrons behind them. The flotilla commander was forced to make a split-second decision, ordering all gun batteries to target the scouts and leaving the rest to the flak guns.
It was a brave but costly tactic. The Stormbirds reached the pickets first, each squadron orientating on a target and thundering in at full power. Explosive shells and multilaser bolts hammered at the oncoming assault craft, but the heavily-armoured Stormbirds pressed on through the barrage. Here and there an enemy shot struck home; engines exploded or cockpits were shattered by direct hits, but the rest continued their attack. They swept in low across the destroyers' upper decks, pummelling their hull and superstructure with cannon fire and melta rockets. Four of the pickets staggered out of formation, their bridges smashed and their decks ablaze.
Seconds later the Imperial torpedoes struck. Seven of them hit their targets, blowing the rebel destroyers apart. The four surviving ships plunged onwards, doggedly trading blow for blow with the scout squadrons. Their void shields blazed beneath a rain of explosive shells and ravening lance beams as they plunged into the Imperial formation. At such close range the gunners could scarcely miss their targets; one by one the shields of the rebel ships failed and the concentrated Imperial fire ripped them open from stem to stern.
But Horus's ships and their veteran crews died hard. They concentrated their fire on the survivors of Destroyer Squadron Twelve, pouring fire into Rapier and Courageous. The void shields of the two destroyers collapsed beneath the onslaught; Courageous died a moment later as a shell found its way into her main reactor room. Rapier fought on a few seconds more, destroying one of the picket ships with her last salvo, before an enemy shell detonated in her torpedo magazine.
Forty seconds had passed since the rebels' first salvo. Captain Ivers, master of the light cruiser Formidable, sent a terse vox to the flagship: the way to Diamat was clear.
'Increase speed,' Jonson ordered, watching the telltales update on the tactical plot. They were less than a quarter of a million kilometres from Diamat now, well within range of the battle group's surveyor arrays, and they were getting positional updates on the enemy fleet in real time.
It had been more than an hour since the initial engagement against the rebel pickets. The Stormbirds had been recovered and were being rearmed for another sortie. Nemiel had expected that the surviving escorts would be withdrawn as well, but Jonson had instead sent the depleted force on a roundabout course that threatened to swing around the far left flank of the enemy squadrons that had weighed anchor and were forming a battle line between Jonson's force and the planet. The rebel transports were still in high orbit above Diamat, surrounded by a protective cordon of eight cruisers.