1st Battle of Thallax Gorge

Mechanicum forces ambush and harass a Son's of Horus column as it pushes it way through what would become known as Thallax Gorge - the traitors manage to hold their positions as increasingly powerful rad storms force the loyalists to cease their offensive push.

++Rad-wastes. Unknown sector++ The sound of grinding tracks and astartes moving in lockstep echoed across the open expanse of the rad-wastes. The fall of Installation Delta had opened the floodgates to the traitor's orbital landings, and now the reinforcements marched through the blasted hellscape to their next destination. Kagaddon insisted that he oversee the march himself, despite his advisors counsel that his 'talents' were best utilised on other fronts. He had grown untrusting of all but Kylik and his Justaerin after the 'faithful true sons of the Warmaster' from the White Scars had turned tail at the first prospect of assaulting the Northern Wall, and now sat in a dark cell in the depths of his ship. His faith in the 1st company reavers had wavered similarly, and had sent one of his old lodge masters to serve as an inspiring orator and, should the needs arise, executioner. As Kagaddon continued to dwell on the failings of his army, a massive blast rocked his land raider transport and the vox opened to the sound of commencing battle. A projection of their surroundings filled the darkened insides of the tank, showing a horde of cult Mechanicum automata and battle tanks, and among them, lobotomised astartes of the 10th, 7th, and 16th. The legionaries accompanying Kagaddon, not his usual ensemble, grimaced at the sight. "They're using astartes. Our brothers. cannibalised them". Kagaddon shut off the projection and glared "If that is enough to unnerve you I question if you should bother leaving this transport." He spoke again, this time addressing his battallion "Sons of Horus. They, who butchered our brothers, seek to stop our march. Give them no quarter-For the Warmaster".
  A sea of lumbering tech-thralls made their way toward the now dug-in Sons of Horus. It was clear that whatever Magi controlled them had one goal; to push the newly arrived forces back through the irradiated deserts on a wave of expendable bodies. The Mechanicus construct that had battered Kagaddon's raider, a Thanatar class automaton, now charged headlong into it, free from any higher control.
  It pounded at the tank, and it began to crumple and stutter. Nearby, squads of astartes took fire from a range of esoteric weaponry, damaging their suits and exposing them to the full effect of the radation saturating the ground. Their enhanced bodies prevented them from death, but it ravaged them all the same, and were forced back. A pair of dreadnaughts, all but immune to the plagues of rad-poisoning, stormed into the lines of enemy astartes, pinning them sheer meters away from their objective. Elsewhere, the black-clad reavers surged out from their carrier and toward the enemy, the centurion among them bellowing "Brothers! Slaughter the false kin! Flee- and you shall die by my hand!". There was an uproar from the crowd of black armoured legionnaires, before they slammed into the line of automata, and butchered them effortlessly, falling over each other to claim a kill. Kagaddon too now led his men into combat. The Thanatar which had began taking hits from lascannons across the field, was mobbed by a squad of terminators, who began to shred at exposed wiring and servos with their lightning claws. The giant fell, and the explosion that followed washed over the terminator's protective fields. All at once, across the Mechanicum army, thrall, automata, and battle tank alike, moved in unison. A Triaros conveyor alighted it's electrified ram, and forced the reavers back through a canyon, whilst Thallax automata took positions above them, ready to fire. Another group of Thallax turned and trained their guns on Kagaddon and his squad, but a bright flash heralded the arrival of the Justaerin, called upon when the battle had broken out. Mass bolter fire shielded the commander as he fell away from combat, but the return fire took down many of the elite warriors.
  "Hold the line!" a shout went up from the reavers. As they prepared to engage with the mechanicum, something in them changed, like a switch turning on. Cybernetica began to back away, firing as they went, whilst thralls dragged away the corpses of the Sons of Horus' dead. The withering fire made it impossible to follow up, but the Sons of Horus held their positions. It appeared to all as if the Mechanicum had conceded. Had the Magos calculated a loss? Had he achieved some hidden objective unknowable to all but him? The questions would linger in everyone's minds in the hours to come, as they re-organised, and prepared for the long march once again.