Campaign Fluff: The Ancient Darkness

Brother Ignatius staggered as another bladed arm struck his helmet, his combat knife flashed in response drawing a splash of Xeno’s blood.
The Ork force had been mostly blunted but the Tyranid’s were all over them, he and his remaining battle brothers had been overtaken by a sea of small vicious creatures and bladed armed Gaunts. Victory was an impossibility now, they had to pull back and conserve their gene-seed as losing it to these creatures would be unthinkable.
How could this even be, clearing an Ork infestation would be challenge enough but having the Tyranid see them as their own! What kind of monster had those researchers created. To make matters worse the Tyranid’s seemed driven by their unrelenting Hive Mind even without Norn Queens or any higher command organisms. Imperial doctrine listed the elimination of these “Leader-Beasts” as highest priority with their destruction sowing chaos and discord among all their lesser forces. This group however seemingly had no such organisation, had they somehow adapted beyond the need for it or was there a missing piece of the puzzle?
Ignatius suddenly felt a twinge of pain as one of the bladed arms of his foe punched through his torso, his armour quickly dulling the feeling and his arm already in motion tracking his attacker with his bolter, one quick burst shredding the cranium of the creature.
Comms chatter was virtually gone, most of the battle group had been destroyed and Ignatius lead what remained of his squad back, they needed to make it to the plateau for extraction and meet up with the Thunderhawk there to get back to the firebase.
He could see the Ork leader, thundering around on his ramshackle excuse for a vehicle, howling in joy, revelling in the bloodshed. He was brandishing his latest trophies, pieces of the armour of the Sisters sent to aid them.
Then the sun was blotted out and the ground shook with a terrible impact, turning his head he saw it, a monstrous winged creature with huge bladed arms like the one that lead the attack on the research station. The creature towered over him clearly scarred by the battle, it’s carapace was cracked and shattered in places with it’s black ichors seeping from the wounds. Ignatius swung his bolter round tracking a weak spot near the neck, but the creature moved faster than he ever thought possible. Scythed arms lashed out slaying two of his battle brothers before they could even raise their weapons while one struck severing his arm at the elbow.
Casting his blade aside he reached for the Krak grenades at his belt, if he could detonate them he might yet take this beast with him. The world slowed as he reached, sparing a glance at the creature as it tore apart the last of his squad it’s eyes gleaming, but there was no emotion, simply the instinct and the deeper sense of something ancient as the stars themselves staring back from behind them as the creatures head whipped forward with it's razor toothed mouth spread wide…