The bolter he carried clicked empty and he dropped it into the effluent, reactivating his power fist as two Imperial Fists rushed him. He killed the first with a mighty punch to the head, and the second with a reverse stroke that tore out his throat.

Another two warriors charged. Forrix roared in battle fury as he felt the blade of a crackling sword rip through his armour, between his ribs and into his primary heart. Angrily he slammed his fist down across the blade, wrenching it from the Space Marine's grip before removing his arm with a backhanded blow. He shoulder charged the other warrior, crushing his helmet against the tunnel wall before disembowelling the armed Space Marine with his power fist.

Gunfire hammered him and he felt the bone shield within his chest cavity crack as a bolter shell exploded within the ceramite plates of his armour. He dropped to his knees as the Imperial Fist closed the gap, firing as he advanced. Forrix tore out the sword protruding from his chest and hacked the warrior's legs out from under him, pitching him face-first into the waste matter.

He pulled himself to his feet as more bullets hammered him. A grenade splashed next to him and he hurled himself back as it detonated. Muffled by the water, the blast threw up a spray of liquid and debris, but its lethal force was spent and he was unharmed.

He rose to his knees as another Imperial Fist charged him. A bolt took Forrix high on the temple, blasting a portion of his helmet clear and blood streamed down his face. Something slammed into his visor, ripping the helmet from his head. He felt his jawbone shatter. Bright lights burst before his eyes and he splashed backwards into the water, gagging as the liquid waste poured into his nose and open mouth.

The toxins seared his eyes and blistered his skin in seconds. He lashed out with his fist, feeling it connect with something solid, and scrambled back, lifting his head from the slime. He spat a froth of viscous matter, retching as his body fought against the toxins he had ingested.

He blinked through the searing pain in his eyes, battling to focus as something came towards him. He punched out blindly, but missed and bellowed in pain as he felt the wide blade of another sword pierce his chest, tear through his lungs and burst through the backplate of his armour.

He gripped the sword blade and kicked out, hearing something splinter and a cry of pain. Blindly, he groped in the swirling, bloody water, feeling something thrashing in front of him. Forrix roared and smashed his power fist down upon the shape, breaking it apart in a flurry of crushing blows. His chest burned with hot agony as his secondary heart and multi-lung fought to keep him alive despite the massive traumas his body had suffered.

He heard more shouts behind him, but he had lost all sense of direction in his blindness. Rescuers or killers?

'Iron within!' he bellowed, raising his power fist, the pain in his chest more intense now.

'Iron without!' came the answering shout and Forrix lowered his arm as the warriors of his company swept past him. He heard echoes of bolter fire and roars of hatred, but they seemed to be growing more and more distant with each passing second.

Forrix tried to climb to his feet, but his strength was gone and he could not move.

A tremendous, deafening explosion shook the tunnel. Rocks fell from above him and orange flames briefly lit up the battle-scarred tunnel walls.

He sagged forward, supporting his broken body on shaking arms.

He heard the victory chant of the Iron Warriors coming from somewhere that seemed impossibly far away.

Only then did Forrix allow his elbows to buckle, collapsing him to the tunnel's floor.

In the days following the abortive attack on the Iron Warriors' tunnel system, the morale of the citadel's garrison slumped as it became obvious that nothing they could do would prevent the mine from reaching the walls. Another assault was mounted through the countermine in the Primus Ravelin, but it was repulsed with heavy losses by a strong tunnel guard that never left the mine workings.

Forrix was carried back to his dugout, where he was attended by the Warsmith's Chirumeks. The master of the Iron Warriors made it very clear that their survival was directly linked to that of his war-captain.

While Forrix healed, Honsou volunteered to take over supervision of the mining operations. Kroeger had not emerged from his dugout for days and Honsou wondered what new blood-madness now possessed him. The Imperial Fists had explosively sealed their countermines when it had become obvious that their attacks could not succeed. Once the damage done by the sally had been repaired, the undermining works progressed once more.

Siege tanks now moved up through the saps towards the third parallel, taking their positions in the heavily fortified earthwork. Day and night, trucks laden with shells for these iron behemoths would make the dangerous trip from the campsite, depositing the ordnance in newly constructed and heavily armoured magazines.

Observers watched as embrasures were cut in the earthworks, the soil left in place until such time as the tanks were ready to unleash their firepower against the defenders.

Fresh trenches were dug backwards from the third parallel, equipped with smaller parallels where huge numbers of soldiers could muster, ready to hurl themselves at the walls.

A sense of dread began to permeate the garrison, despite the officers' attempts to raise spirits and boost morale. The sheer scale of the assault soon to be unleashed upon them preyed upon the minds of even the most determined Imperial defenders.

Three days after the attack on the tunnels a terrible rumbling rocked the walls of the citadel, like the beginnings of an earthquake. The ground beneath the fortress heaved upwards and cracks split the roadways throughout the inner walls.

Along the edge of the ditch, a huge wall of fire and smoke leapt upwards as explosives planted there blasted its crest apart, scattering rubble into the ditch and providing a means for infantry to descend into it.

But barely had the dust settled when an explosion of far greater magnitude shook the ground. Wide galleries that ran underneath the curtain wall linking the Destiny Gate and the right flank of the Mori bastion collapsed as vast quantities of ordnance detonated and vaporised huge swathes of the wall's foundations.

The centre section of the great wall groaned as it sagged, the noise swelling as a giant crack split the curtain wall, the sound like a deafening gunshot. Officers shouted at their men to clear the walls, but for many it was already too late as the sixty-metre high wall slid ponderously into the ground, huge chunks of rockcrete shearing away and tumbling into the ditch. Hundreds of men were carried to their deaths and vast clouds of dust billowed skywards.

As more of the wall fell, the speed of its collapse increased exponentially, whole sections of the ramparts toppling into the ditch. The scale of the destruction was incredible and it seemed inconceivable that such a mighty edifice could be so thoroughly annihilated.

By the time the collapse had ceased, almost the entire centre of the wall had been brought down. A great breach some thirty metres wide had been torn in the curtain wall, the rubble from the wall's destruction forming a debris slope that ran from the floor of the ditch to the crest of the breach.

The Iron Warriors had broken open the citadel.