The stairs didn't switch back upon themselves, like in most structures; instead they descended in a long, arcing spiral that wound ever deeper into the earth. The foul presence permeating the air grew stronger with each and every step. Zahariel concentrated on putting one foot in front of the next, recalling the labyrinthine steps that wound through the ancient stone beneath Aldurukh itself. Memories flitted through his mind as he walked; of his initiation into the Order and his long walk through darkness at Jonson's side. Fragmentary images came and went: stone steps and torchlight, the rustle of fabric, Nemiel's presence at his side as they descended a flight of stairs to… where? He couldn't quite recall. The memories were vague and only half-formed, like scenes from a dream. A dull pain swelled in the back of his head as he tried to concentrate on the images, until finally he was forced to push the thoughts away.

More alarming were the cracks that began to appear in the outer walls of the stairwell as they descended deeper beneath the ground. Black roots had forced their way through freshly-laid permacrete more than a metre thick, spreading across the inner surface of the curved walls and spilling black, foul-smelling dirt onto the stairs. Red light glistened on the segmented bodies of insects that wormed and writhed their way among the roots. Ghostly white cave spiders, each as big as Zahariel's hand, rose up from their nests and brandished their long legs in challenge as the Astartes went past.

By the time they reached the lowest levels the stairway was little more than a tunnel of raw earth and dripping plant matter, thick with crawling, chittering life. Strange, misshapen insects, bloated and foul, squirmed amid dense networks of rotting root matter. A long, segmented millipede, nearly as long as Zahariel's forearm, uncoiled like a spring from the curve of a root ball and leapt onto his shoulder, stabbing wildly at the armour plate with its needle-like stinger. He brushed the foul thing away with the haft of his force staff and crushed it beneath his boot.

Still, the squad forged ahead, pressing through the ever-constricting tunnel until Zahariel began to think they would be forced to cut a path with their chainswords. Finally, Astelan and the warrior beside him at the front of the formation came to a halt. The air was stifling, thick with heat and the smell of rot, and the red emergency lights had long since given out. Dimly, Zahariel could sense a vague, greenish glow down and to the right, past Astelan's shoulder.

'We've reached the bottom of the stairs,' Astelan said quietly, casting a wary eye up at the swarms of insect life rustling ceaselessly overhead. 'What are your orders?'

There was no telling what they might find beyond the opening to level B6. Zahariel was surprised the enemy had let them penetrate so far - he'd operated on the assumption that they would encounter resistance almost immediately, which would have at least given him some idea of what they were up against. The time might come very soon when he would have to draw upon his psychic abilities, whether he wanted to or not. He needed information more than anything else at this point.

'Press forward,' he said. 'Drive for the thermal core. It's the largest chamber on this level.'

The chapter master nodded and stepped into the green-lit blackness without hesitation. Zahariel followed with the rest of his squad, bolt pistol at the ready. His feet came down on thick roots and cablelike vines stretching across the floor beyond the stairwell. Draughts of stinking air gusted past his helmet, and the insect noise surrounding the warriors swelled to frantic life.

They pressed on down a low-ceilinged passage for more than a hundred metres, passing numerous cross-corridors as they went. The clinging plant life continued unabated down the passageway, and Zahariel realised the pale green glow came from colonies of bloated grubs that clung tenaciously to the twisted roots. Sounds of restless movement echoed all around them, seeming to grow louder with each passing moment. At one point Zahariel heard the clatter of talons behind a cluster of pipes half-hidden among a network of vines running along one of the walls, but he couldn't catch sight of the creature that made the noise.

'How much farther?' Gideon asked quietly. The warrior's voice was tense. The continual screeching and rustling had the entire squad on edge.

'Fifty more—' Zahariel started to say, just as the air filled with a hideous screeching and dark, armoured shapes burst from the plant life all around them.

He was glancing over at Gideon just as a segmented creature struck downwards at the Astartes from the network of thick pipes running overhead. It was swift as a tree viper but as thick as Zahariel's upper arm, with hundreds of chitin-sheathed legs and a broad head set with a half-dozen compound eyes. In a flash it had wrapped around Gideon's torso and lifted the huge warrior off the ground, lunging and snapping at the back of his helmet with its curved mandibles.

Bolt pistols barked and chainswords howled in the confined space as the squad was set upon from all sides. Gideon twisted in the monster's grip, slashing at its body with his whirring blade. Zahariel blew the creature's head apart with a single shot from his bolt pistol just as a powerful impact struck the back of his helmet and pitched him off his feet.

Zahariel tried to twist his body as he fell, but the creature had his helmet gripped in its mandibles and it was stronger even than he. It drove him face-first onto the floor, wrenching his head left and right as it tried to crack the helmet he wore. Something sharp jabbed at the back plate of the helmet like a dagger, trying again and again to punch through the ceramite. Warning icons flashed before his eyes, informing him of his suit's failing integrity.

With his elbows and knees on firm ground the Librarian flexed his augmented muscles and managed to twist onto his right side. His force staff was pinned beneath him, but he was able to aim behind him at the creature's thrashing body. It took three bolt pistol shots in rapid succession to blow the thing apart, showering him with fragments of chitin and reeking ichor. In the muzzle flashes of his pistol Zahariel could see three more of the monsters rearing up from the walls like snakes, their mandibles clashing as they prepared to strike. Without hesitation, he summoned up the full force of his will and unleashed the psychic fury of the warp.

He had practised the attack countless times under Israfael's tutelage, but the sheer intensity of the energy coursing through him took Zahariel by surprise. It roared through him like a torrent, far stronger and easier to grasp than he'd ever experienced before. A nimbus of crackling energy surrounded the Librarian; he felt each and every vein in his body turn to ice, radiating from the cables of the psychic hood at the back of his skull, and the three creatures were engulfed in a torrent of raging fire that coalesced from the very air itself. They burst apart in the intense heat, their carapaces exploding from within.

Zahariel gave a shout of triumph and surged to his feet. Skeins of crackling lightning played over the surface of his staff, and icy power raged along his limbs. For a dizzying instant his awareness sharpened to a supernatural degree, reaching into dimensions beyond the understanding of ordinary humans. The permacrete and metal of the corridor faded into near-invisibility, while living matter was etched with vibrant clarity. He could see the layers of root and vine blanketing the walls and ceiling, and every one of the thousands of insects living in their midst. He could also see the score of worms surrounding his squad, wrapping about the warriors and biting at their armoured forms.

Worse, he could see the awful, unnatural taint that pulsed through it all. It stained every living thing in the corridor around the Astartes, corrupting them like a cancer. A cancer that seethed with awful, otherworldly sentience.