A whistle blows somewhere up the line.
Everyone is tense, I can feel the breath of the man behind me on my neck, scalding hot in the frigid cold. My hands flex on the ladder, awaiting the inevitable order.
I hear a sob next to me. A boy from our platoon, likely not even in his twenties yet, stands gazing at the wad of skin and raw flesh hanging from the frozen ladder before him, a torn portion of his own ungloved hand.

Every distant detonation shakes the ground, hard enough to make some men lose their footing. The smell of urine stabs the frozen air, and another whistle blows, closer this time.
"Duck!" a voice shouts, and as one, those on the ladders above us bow their heads as a trio of Leman Russ tanks roll over the trench top. Dead weight slams into me from above, sending me crashing into the mud.
The mud of the trench floor sucks me down, submerging me.
I am blind, my eyes stung with the murky, rancid mix of mud and urine and something warm with a metallic tang.
The foul soup seeps into my mouth as I try to breathe, forcing its way down my throat. I gag and choke, eventually managing to throw the dead weight off and rise.

At last I can see what almost damned me to an Emperor-forsaken death in the trench floor.
The man above me, a sergeant by his pins, had failed to duck his head in time, and as a result the Leman Russ, completely oblivious to its casual slaughter, had crushed his head beneath its tracks, crushing it into a fine pate laced with bone and sinew.
I watched with disgusted fascination as his body continued to twitch and spasm below me.

The whistle blew once again.
I could see the platoon next to ours filter up and over the trench top.
Some of them slipped and fell, others were shot in rapid flurries of lasfire and punctuated explosions of solid rounds.
All were trampled underfoot by the stampeding beast of the Imperial ranks, a machine poked and taunted forward by the cruel lashes and blades of the commissars behind.
I clench my bladder to keep from wetting myself. My stomach turns inside me, aching to throw up. But I can't vomit. I've already defecated my rations today. Some of the men melted snow to drink, but this world is not ours, and those who drank the snow-water became consumed by sickly boils, some as big as a man's fist.

That damn whistle blows again, a rudimentary and primitive replacement while vox is down.
This time it blows shrilly in my ears, causing them to ring. The battle chants and prayers of those around me come muffled. I scramble to the top of the ladder, sliding over and running forward blindly, lasrifle in hand, spurred on into the hellscape of mud and shell holes by the knowledge that if I stop, if I pause for one moment, I will be consumed and stamped to death by the sheer number of desperate feet charging behind me.

Lasfire peppers our ranks, I squeeze my eyes shut as the rounds pass over and around me. By now the rolling death machine of our own lines has caught up with me. I glance to my left and see bodies. Guardsmen, Officers, Commanders, Commissars, some of them dead, their limp, puppet-like forms carried forward by the fervor of those behind.

Something snags my leg, it hurts like hell and I can feel something wet slapping against my ankle, but I can't stop, if I stop, I die. So I continue on, every labored step takes more effort than the last.
I can see the line of Leman Russ tanks before us, carving a path, their cannons recoiling with vicious firepower. The noise takes a second to reach my eardrums, but when it does it is deafening.
I'm practically dragging my own leg now, it hurts so much, but I know I can't stop.
The Emperor watches me, so the Commissar says, and if he is dissatisfied he will surely release me for darker powers to judge.
Tears streak my face, hot only for the briefest of seconds before the cruel cold freezes them to my skin.
Fire ripples the air as the Leman Russ to the right explodes in an incandescent bow wave that warms me to the bone and fills my nostrils with the scent of singed flesh and hair, throwing me to the ground atop the corpse of some other poor sod.
I try to find my feet, but those behind me charge forward, kicking and slipping and trampling over my prone form. I feel something snap in my chest, breath comes raggedly to me now, but there is no pain.

My leg, on the other hand, is still on fire, but I dare not look, already knowing what I will find. A chunk of my thigh, not snagged but torn away in a hail of enemy fire, swinging against my ankle by a flap of torn skin and screaming nerves.
Finally, the worst of the stampeding Guardsmen pass, and I can at last push myself up. The corpse beneath me wheezes as his stomach caves in beneath my hands. His blood smears my uniform, along with the urine and excrement and every other substance accumulated since my fall in the trenches.

I look up, painfully aware of the sudden silence that surrounds my immediate position. The percussive thuds and whines and screams of war echo around me, but fog obscures my vision so thickly I might as well be a continent away.

The world around me is suddenly bathed in red, and my hearing is assaulted by a grinding engine noise. The Leman Russ reverses towards me at an astounding rate, fleeing some terror as yet unseen by my eyes.
My leg, finally realizing it should not be able to support my weight, collapses beneath me. The ground vibrates with the sickening motion of the reversing tank.

Instincts older than my body and more primal than my mind kick in, and my fingers claw the mud beneath me, trying to find some purchase to drag me away from the merciless treads grinding towards me. I kick with my good leg, finally managing to gain some purchase, and propel myself to the side.
I extend my fingers to drag myself further, but the throaty engine noise behind me is suddenly accentuated by a sickly snapping noise, a blinding pain in my one good leg, and an ear-wrenching scream which I realize is coming from my own mouth.

The Leman Russ continues reversing, completely oblivious to the fate it just dealt me. Booted feet follow, precious few compared to the horde that had urged me forward.
A Commissar follows, pot-shotting those stragglers foolish enough to remain in his vision, vainly trying to restore some semblance of order in the futile hope that the battle might yet be salvaged.

"Help!" I cry out, heedless of how high and feeble my voice sounds, "Emperor's sake, help me!"
Nobody stops, nobody even looks.
So consumed in their desire to flee are my comrades that none can bear to carry my weight.
Another Guardsman runs past, this one stops, stares at me.
"Help..." I utter feebly.

The man moves to help me up, but a stark scream nearby changes his mind swiftly.
"Sorry, Edrin." He murmurs to me before hurrying on, not even doing me the mercy of shooting me.
My heart sinks and my lip trembles as darkness clouds the corners of my eyes.
I don't want to die.
Ahem, that's how.
The mood is important, the desperation and a good sense of futility also help.
Try to keep the enemy obscured for as long as possible. A good majority of deaths in war happen long before your soldiers come anywhere near the enemy troops.

Most importantly, make it seem as though death is described as more of a morbid curiosity or just a feature in the scenery, rather than romanticizing or martyring it in any way.
In other words, no "brave heroes" and no "poor lost souls".

Death as something that is detached or just a part of the landscape is often more impacting to players, because of how it implants the notion that nobody cares what happens to the guy next to them when you're in the meat grinder.

Overall, describing the desperation and the poor state of the men does more than just describing a mountain of corpses. Of course, there's always going to be a sperg or two who just don't give two fucks, but you can't please everyone.