The shrieking continues, causing everyone still alive to cover their ears- even Peg-leg, a simple servitor, tries to do so, tottering back towards you, away from the sound. The marines habitually fall back, clustering up, back to back, bodies shivering with pain as the sound continues. A few of them yell back, shouting litanies in the face of the sound, throwing back corruption with purity and hate.
The orks roar back, of course. Ain't no squawking git gonna out-noise the WAAUGH.
The grate bursts open, and something massive and heavy falls through it, hitting the ground with a meaty thud that topples poor Peg-leg, and everyone else near it. The unexploded corpses of the cultists suddenly sing with praises of corruption and scrapcode, illogical maths and charts in dimensions not meant for man to hear.
It is a terrible thing- a daemon half-real half-unreal, a thing still thought as much as flesh. It contracts, the manufactured body of scrap and electronics shivering and warping as the thought of the daemon tries to coalesce. Then it unfolds again, revealing great shining wings of circuit board, feathers made from glass and bones, tied with sinew and wire.
The body is carefully-stitched muscle and fibers, eyes made of lights and barely-contained hatred, a massive gaping hole in the chest writhing with the invisible energies of the warp and the pale blue arcs of electricity. It opens it's beak-maw of metal plating and jawbones, and roars in anger. It has no lower body - the whole thing feels half-made, not finished, a little sloppy.
'My appearance has been delayed too long! No longer! Here I will make my legs from your BONES, Astartes! Here I will make my spine from your WEAPONS, ASTARTES. HERE I WILL MAKE MY STAFF FROM YOUR SOULS, ASTARTES. I WILL MAKE YOU INTO ME, AND WE SHALL WRITHE IN THIS PLANE FOR EVER, UNTIL MY LORD HAS QUASHED YOUR CORPSE-GOD!'