Just watched "Deathwatch" based on this thread, and to be honest it felt like actual historical WWI was incredibly horrific already, without even adding supernatural elements.
Vast stretches of stinking mud, rain, fog, barbed wire, putrescent corpses, rats, flies, trench foot, gangrenous wounds, and that's before you even factor in the artillery, snipers, gas, enemy raids, incompetent command, and ever-present threat of death - either swift and brutal or lingering and painful.
Be sure to play up the fundamental misery of trench warfare. Everything is perpetually damp (not fun with woollen uniforms), the trenches stink of decaying flesh and human waste, rats get on fucking everything.
Then add things that could easily be hallucinations brought on by stress and lack of sleep, low-key, subjective and dismissable by those in command.
The trench walls seem to crawl with unseen things just below the surface; the bloated and decomposing faces of friends and comrades stare up from the reflections in puddles; grasping fingers reach from between the duckboards before withdrawing; rations start to taste of death and rot; they glimpse what looked like a human thumb in their mess tin; unmasked soldiers are seen walking unaffected through the murk during a gas attack; a sentry sees a horde of rats sweep down the trench, flowing around his ankles like a tide of matted fur before passing him by - what are they fleeing?