The units under Cawl's command swooped out of the defence box at his urging, falling into a close arrow formation. They moved with perfect synchronicity, only a few metres apart. On his displays, icons flashed indicating where Aspertia wanted him to go, but Cawl had read the situation and was already on his way.

The shoal of xenotic lifeforms was ahead.

A gaseous bladder was their principle feature. The density of the atmosphere allowed a certain extravagance of form; these were not diaphanous creatures, but solid, well-armoured hunters.

Stable dathrates armoured the upper surfaces of the flight bladder and the multiple-jointed legs that trailed behind it. Underneath all that was a sagging mouth and several boneless nozzles that projected gas in a high-velocity stream, allowing the creatures to control their motion with remarkable efficiency. They surged about like darting birds even in the thousand-kilometre-an-hour winds of the planet.

Nobody knew what the things were, if they were intelligent or simply animals, or if they were native or alien to the world. The forge world's xeno-genetors itched to get hold of a specimen, but it had so far proved difficult. After they were killed most fell into the raging turmoil of the planet's interior. The rare specimens snagged by the scows had disintegrated during transit back to the stations. Their clathrate armour was hard enough to deflect autocannon rounds, but after death it destabilised rapidly, and the soft tissues quickly followed. They had no utility to the Imperium, so studying them was not a priority, and of interest only to xenos-obsessed eccentrics. The effort to gather a specimen distracted from the cloud mining. All the Trisolian Mechanicum required of the xenos was that they stay away or have the grace to die quickly, and so the xeno-genetors remained in ignorance of their mysteries.

The cloud creatures were far more nimble than the robots, and swiped at them with limbs that were brutal in defiance of their delicacy. Unit One of Maniple Nine lost a fan pod, and though its shepherd overrode its innate programming, took direct control and fired off the machine's void engines in an attempt to stabilise it, it went into a spin and plummeted into the roiling clouds.

Cawl watched his robots carefully as they drew firing lines on the nearest clutch of the xenos. Autocannon fire flaring with phosphor traced lines across the sky. The machines' limited brains could follow these ranging markers easily, and missiles quickly followed.

A creature rocked under the impact of Cawl's assault. Clathrate armour collapsed. A missile pierced soft flesh beneath and passed out of the other side before detonating in the air. The creature was mortally wounded and lost altitude, leaking a stream of bright gas from its breached shell.

'Do not allow them within the cordon,' said the domina in her multiple tongues. 'Plug the gap, hold the line.'