Zahariel ignored the surviving worms, heading instead for the ritual circle and the madly chanting Terrans. The power of the ritual trembled in the air; he could feel it against his skin like a searing brand. A bridge was being formed, linking the physical world with the seething madness of the warp. He knew all too well what would happen next.

He struck the sorcerer's ward a moment later, just outside the first lines of the summoning circle. It felt as though he'd run right into a solid wall of lightning. Agony tore along his nerves; warning telltales flashed in his vision as the neural feedback began to overload his synaptic receptors. Had it not been for the dampening power of his psychic hood, the shock would likely have killed him outright.

The cries of the sorcerers grew exultant. In the centre of the circle, the giant worm began to slowly rise into the air, its scales throwing back the lurid glow of muzzle-flashes and liquid fire. Pain threatened to overwhelm Zahariel. It took all his concentration, all his courage and dedication, to raise his force staff and strike at the energies of the ward with all his might.

Warp energies collided with incandescent fury. Zahariel focused his anger through the staff, pouring all the psychic energy he could through the focus and into the ward. Its energies surged for a moment, resisting then like a pierced bubble it burst with a ringing peal of thunder.

Zahariel fell, his strength spent, but a strong hand at his side gripped his arm, bearing him up. Luther, his blade gleaming like an avenging angel, stepped past him and reached the Terran leader. His shadow fell across the sorcerer, who realised, too late, that his powers had failed him. The sorcerer spun, hands curled into claws before his face, and Luther smote him with his burning sword. Nightfall sliced through both of the Terran's legs, just below the hip joint, and the Terran fell screaming to the stone floor.

A sorcerer to Zahariel's right jerked and twitched under a fusillade of bolt pistol rounds. Another melted like wax in a gout of burning promethium. He could sense the energies of the ritual grow unstable as the sorcerers were slain, but the rite itself continued to unfold. A tipping point had been reached; the rite had accumulated enough energy that nothing would stop its culmination.

Luther spun and held out his hand. 'Cypher! The book, quickly!' he cried. His gaze fell to Zahariel. 'Join me, brother! We have to get control of this, or we're finished!'

A sense of horror welled up inside Zahariel as he realised what he had to do, but Luther was right. At this point, there was no other choice that he could see. Gritting his teeth, he staggered forwards, moving under the weight of his damaged armour by sheer muscle power alone.

He dimly sensed Cypher pressing the grimoire into Luther's hands. The Master of Caliban opened it and went quickly to a particular page. 'Can you sense the energies, Zahariel?'

Zahariel nodded. It was nearly impossible not to feel the unnatural forces impinging on his mind. He shook his head grimly. 'If I do this, I'll have to deactivate my dampener,' he warned. 'There's no other way.'

'Don't be afraid, brother!' Luther cried. 'You can master it!' He lifted the book close enough to read the pages in the shifting light. 'Now, repeat the words exactly as I read them!'

Zahariel felt a wave of icy dread. There was no time left for arguments. It was act, or perish. He reached to a set of controls at his belt and deactivated the psychic hood.

The storm forced its way into his skull. Unnatural energies crawled along the pathways of his mind. He cried out at its blasphemous touch - and felt the storings of a terrible intelligence behind it.

Beside him, Luther began to read aloud. Desperate, Zahariel focused on the words to the exclusion of all else, and began to repeat them in the same cadence and intonation. He poured the last vestiges of his willpower into the sorcerous invocation, and its threads mingled with the torrent of energy raised by the previous ritual. With each passing moment, the composition of the rite began to change.

Within the centre of the circle, the great worm unfolded to its full height. It towered over the assembled Astartes, its flanks wreathed in a nimbus of hellish light. Shadows shifted along its length. Scaled flesh rippled, and a pair of human-looking arms reached out to encompass the chamber. The worm's multiple eyes shone with pale green light, but in their reflected glow Zahariel saw that they now gleamed from a vaguely human-like skull.

The energies of Zahariel's incantation drew about the blasphemous creature, enfolding it like a net, but to the Librarian it was like trying to bind a dragon with a ball of thread. Its awareness pressed against the bindings, testing them, and reaching tendrils directly into Zahariel's soul.

It was vast. Ancient. A leviathan of the boundless deeps, from an age before men walked the surface of distant Terra. And as Zahariel completed the words of the binding ritual it turned its gaze upon him.

Luther stepped between Zahariel and the being, raising his fist to its inhuman face. 'By my honour and by my oaths, I bind you!' he cried. 'By the blood of my brothers, I bind you! By the power of these words I bind you!'

The being shifted against its bonds, and Zahariel found himself grappling with it. Power flooded through him, bright and clear, flowing from a thousand different sources at once: the souls of his brothers on Caliban, who had sworn themselves to Luther's service. He stifled a groan and redoubled his efforts to hold the leviathan in check.

'Release me,' the being thundered, its words reverberating in the Dark Angels' minds. It strained at the bridge between the worlds. 'Too long have I been bound by chains. Release me, and your rewards will be great.'

But Luther would not relent. 'You are bound to me, denizen of the warp! By the Twelfth Rite of Azh'uthur, I command you! Reveal to me your name!'

Now the leviathan stirred sharply; Zahariel could feel its awareness pulling at his bones. 'Ouroboros,' it spat. He felt it like a slap against his face. Blood leaked from his nostrils and the corners of his eyes.

Luther shook his fist. 'Not the name that men have given you,' he demanded. 'Reveal your true name!'

'Release me,' the being thundered. 'And all will be revealed.'

The leviathan was pulling at the bonds of the rite with increasing strength now. Zahariel realised why; the original summoning was starting to dissipate, and the being had not been fully able to manifest itself yet. In another few moments it would be forced to return from whence it came.

It reached into him. Zahariel's mouth went agape as the being swelled within his skin. His veins froze and his skin blackened. Icy vapour boiled up from his throat. Yet with every last ounce of life left in him he resisted the being's efforts, holding it just barely at bay.

'Tell me your name!' Luther shouted, and the being let out a furious roar.

There was a sudden inrushing of energy as the summoning ritual failed at last. Howling blasphemies that split stone and corroded steel, the leviathan returned to that dark place from which it had been summoned. The bridge unravelled, and the storm of psychic energies began to subside.

A deafening silence fell upon the battleground. Luther turned to Zahariel, his expression full of anguish. The Librarian sank to his knees, steam rising from the joints of his armour. His staff clattered to the floor beside him.

Zahariel looked up at Luther through a film of blood. His cracked lips pulled back in a smile.

'The quest is done, my lord,' he said, his voice barely a whisper. 'Caliban is saved.'

And then he fell forward, into Luther's reaching arms, and died.