A blast from his plasma pistol burnt the flesh from the face of a black-robed Shinarii devotee, hurling his body backwards so that the spiked mace he had been wielding above his head fell backwards onto the skull of the madman following him. Another six sabre-waving cultists fell to the bolter fire of the rest of the squad.

They were almost there. Soon the entire Citadel would be cleansed of the Shinarii's obscene presence and the slow process of re-consecrating it to the service of the Emperor could be begun: it would take an army of Imperial priests months to complete the work that would be required, such was the scale of the desecration.

A lance of white-hot pain drove through Dagan's brain, blinding him and making him cry out in pain. Unable to help himself he crumpled to his knees. He was dimly aware of another of his battle-brothers moving towards him to offer assistance. Then that Space Marine was on his knees too, crying out in pain.

Dagan tried to stand but simply raising his head made him feel dizzy and sick. He realised that blood was running from his nose, inside his helmet. He forced himself to open his eyes, ready to confront whatever it was that was determined to bring about his end.

Gliding into the chamber was another black-robed cultist creature - for creature it was to Dagan's mind, for how could something like this be considered human? The cultist was stripped to the waist, chains binding its torso. The symbol of an eight-pointed star was carved into the visible flesh beneath. The creature was completely hairless, the frontal lobes of its brain having grown in such a distorted manner that the cultist's whole skull had been swollen out of shape.

Its pupil-less eyes shone with an intense white inner light and an aura of coruscating energy enclosed its entire body. Dagan could feel the static charge prickling on his skin. The creature was levitating a full half metre above the floor of the chamber.

'Greetings, noble warrior.’ the blasphemy cackled.

A witch-cultist, Dagan realised, a rogue psyker. They were the bane of the Imperium and the chosen prey of the Ordo Hereticus. Without the powers of the psyker the vast Imperium of Mankind would be as nothing, unable to send messages or ships across the immeasurable vastness of space. But the untrained psyker's mind was something that could ultimately bring about the Imperium's destruction.

He had to destroy it, now, while he still could.

A second stab of mental pain, like the agonised death-scream of a thousand dying penitents, burst inside his skull. The hulking Space Marine doubled up and vomited across the shattered mosaic floor before him. He could hear the witch-creature still giggling inanely as it drifted closer, accompanied by the whimpering of his mind-tortured brethren.

Then the sickening laughter stopped abruptly.