The fact was, he was sorely tempted to defy Luther and order the Astartes into action. Technically, it was within his authority as Luther's second-in-command; with the Master of Caliban in seclusion, the decision was his to make, but doing so would betray his oaths of obedience to the Emperor and to the Legion. And yet, what if Luther was right, and the real danger to Caliban was from the Imperium itself? If that were true, then his oath to the Emperor was based on a lie, and counted for nothing. He didn't know what to believe at this point. The things he'd witnessed at Sigma Five-One-Seven had shaken his faith to the core.

In all his life, Zahariel had never lacked for certainty. His faith in himself and his cause had been unwavering. Now it seemed like the very foundations of the world were quaking beneath his feet. If he wasn't careful, his next step could well be his last.

Overhead the storm raged, mirroring the turmoil in Zahariel's mind. He drew in a deep breath and channelled his frustrations into a mental summons.

'Show yourselves, you Watchers in the Dark!' he shouted into the raging wind. 'Long ago, I pledged my sword to you, to stand against the same evils that you did. Now I see the truth; this whole world is corrupted, and now my people are in dire peril.'

Another searing flash of lightning answered his mental summons, banishing all but the deepest shadows and etching the courtyard in sharp relief. But this time the brilliant light did not fade; it deepened slightly in colour, from a harsh blue-white to a more silvery hue, like moonlight. Zahariel no longer felt the touch of rain on his cheeks, and the howling wind seemed strangely muted, almost plaintive in its howls. Then he saw the three, hooded figures standing at the centre of the spiral. They were garbed like supplicants, wearing a surplice whose colour seemed to constantly shift from black to brown to grey and back again. Their heads were covered by voluminous hoods, their faces hidden by darkness. Their hands were tucked inside the sleeves of their surplice, so that not one centimetre of flesh could be seen.

The Watchers in the Dark weren't human. Of that, Zahariel was certain. This was the form they chose to show him, because he was quite certain that the sight of their true nature would very likely drive him mad.

One of the three spoke - Zahariel could not be certain which one. Their voices were like a complex skein of whispered sounds, woven together into the semblance of human words.

You know nothing of truth, Zahariel, the watcher said. If truth and falsehood were so simple, our ancient enemy could never find its way into a human soul.

'I know what is right and what is wrong!' Zahariel shot back. 'I know the difference between honour and dishonour, loyalty and treason! What more does a man - or an Astartes - need to know?'

He is blind, said one of the watchers. He has always been thus. Kill him, before he does more harm than he knows.

Though the watchers were diminutive creatures by Astartes standards - each one barely more than a metre in height - Zahariel could sense the mantle of psychic energy that surrounded each of them, and knew that they could snuff out his life as easily as a candle flame. But he was in no mood to be cowed by these beings, not when the future of Caliban was at stake.

'Perhaps that was true once, but I have learned a great deal since the first time we met,' Zahariel countered. 'You're not ghosts or malevolent spirits, as the forest folk once believed. You're a xenos species that has been guarding something here on Caliban for a very long time. What is it?'

Something mankind was not meant to trifle with, one of the watchers hissed. It has ever been thus. Your kind is too curious, too grasping and ignorant. It will be your undoing.

'If we are ignorant, it's because beings like you withhold the truth from us,' Zahariel shouted. 'Knowledge is power.'

And mankind misuses its power at every turn. One day humanity will kindle a fire they cannot control, and the entire universe will burn.

'Then teach us!' Zahariel said. 'Show us a better way, instead of sitting back and waiting for disaster to fall. If you don't, then you're just as much to blame for what happens as we are.'

The three beings stirred, and a wave of psychic power rolled away from them like a cold wave, engulfing Zahariel and freezing him to the core. The shock of it would have stopped an ordinary man's heart; as it was, the Librarian's circulatory and nervous systems struggled to keep him conscious. Yet he refused to be cowed by their expression of pique.

'You said to me, long ago, that this evil could be fought,' he said. 'Here I stand, ready to fight it. Just tell me what I must do.'

The watchers did not answer at first. They stirred again, and the ether was charged with pulses and ripples of invisible power. He sensed that they were conversing somehow, on a level too rarefied for him to perceive.

After what felt like an eternity, the ether stilled once more, and one of the watchers spoke. Ask your questions, human. We will answer what we can.

The admission surprised Zahariel, until he remembered that the watchers had once admitted that they were a part of a larger cabal, dedicated to battling the most ancient of evils. For the first time, he perceived that there were limits to what these potent beings were capable of doing.

'All right,' Zahariel began. 'How long has Caliban been tainted by this evil?'

Always, was the watcher's wintry reply.

'Then why have no Calibanites succumbed to its touch before now?'

Because of our efforts, you foolish human, another watcher said. Zahariel was coming to recognise tonal differences between the beings now, though he still had no clear idea which voice belonged to which body.

And, ironically, by the great beasts themselves, another watcher said. They were born of the taint, and lingered near the places where its corruption rose close to the surface. They killed nearly all of the humans who strayed too close, and those few who did survive were ultimately slain as warlocks by your own people before they could grow too strong.

A sudden chill raised gooseflesh on Zahariel's skin as a memory returned to him from long ago. He remembered standing in the great library of the Knights of Lupus, listening to the bleak words of their doomed master, Lord Sartana… The worst… of all this, is the Lion's quest to kill off the great beasts. That's the real danger. That's the part we'll all end up regretting.

And now the Terrans had come, cutting away the forests and forcing their way into the most inhospitable parts of Caliban in search of resources to feed the Imperial war machine. 'The thermal cores,' he mused. 'They sank the thermal cores deep into the earth and released the taint in the Northwilds.'

And now others feed it with fire and slaughter, a watcher added.

Zahariel nodded, thinking of the pile of corpses at Sigma Five-One-Seven. Many of them had doubtless been provided for the worm queen to lay her eggs, but others - likely the entirety of the Calibanite labour force - had been offered up as a sacrifice, to add power to the ritual and focus the energies that the sorcerers unleashed. If they managed to tap into the horror and bloodshed being unleashed by the rebels, what terrible things might they accomplish?

In their own way, the rebels were more dangerous than the sorcerers themselves, Zahariel realised bleakly. And tragically, their cause wasn't entirely unjust. The Imperium did, in fact, pose a grave danger to Caliban - just not in the way that many of them suspected.

Except for the old knight, Sar Daviel. He knew. Zahariel remembered his last words to Luther.

The forests are gone, but the monsters still remain.

Zahariel suddenly understood what had to be done. He turned to the watchers and bowed his head respectfully. 'Thank you for your counsel,' he said gravely. 'You have my word that the wisdom you've shared will be put to good use. I will save Caliban from destruction. This I swear.'