The primarch gestured to the broken device, ‘Remove this,’ he ordered the Sisters of Silence. ‘Make sure all the pieces are gathered.’

They moved forwards. A fusion lance was brought up, and lascutters, and they began to dismember the remains. Guilliman watched them begin work, then turned his back. Mathieu looked at him in adoration.

‘I advise you to leave, militant-apostolic,’ said the primarch. ‘This place is not safe for you. You have been here long enough.’

Mathieu’s face changed from rapture. He frowned. ‘My lord, I…’ he blinked and pointed.

‘Lord Regent!’ gasped Tigurius. ‘Something is coming!’

Guilliman looked back at the clock in time to see the first Sister die. A stabbing blade shot from the workings she was cutting into, running her through and lifting her three metres into the air on a pillar of brass that moved and coiled organically, reforming itself into something deadly.

‘Daemon!’ screamed Maxim.

A pulse of energy boomed out from the wrecked clock, and a foul wind blew in all directions. The carving of the Emperor on the wall rattled and banged against the vandalised stonework, came loose from its fittings and crashed down to the floor. The Librarians roared with effort, their aegis hoods blazing with psy-amplification.

‘We cannot hold it closed – a rift is forming!’ one shouted. He fell back, staggering, whips of psychic feedback cracking out and lashing the cathedral pillars.

The Custodes bowed into the wind, their boots scraping on the floor as they were forced backwards by the unnatural gale. Guilliman planted the Sword of the Emperor in the ground and knelt behind it. Flames streamed from its edges around him, forming a golden shield.

A crack of thunder presaged the opening of a tear in realspace, and a heaving, gelatinous presence poured through, splashing into the broken clock. Long streams of it wrapped themselves around the workings, its touch turning them green and dull with decay even as it bound them together, remaking them and pulling them into a tall, detestable shape.

A black, oily skin formed over the clockwork and broken stone. The daemon rose up, gathering the stuff of the clock and the menhir into itself, and taking on humanoid form. Organs of whirring cogs sank into its chest. Ropey muscle moved under the shining black skin. Where the metal and stone showed in its form, they were corroded: brass and bronze becoming green, fused lumps, and the rock pitting, though it glowed brighter and brighter.

Forearms grew and grew, the fingers becoming long, backward-facing spikes, like the wings of a bat. A short powerful rear pair of legs burst from the back of the mass. Huge shoulders grew in seconds, unnatural bones cracking as they grew at pace.