Gothic Horror

It appears to me many artists take the idea of "Grimdark Horror" setting as a low-brow approach to quality: resulting in crude, ugly and disgusting monsters/locations stamped en masse: the hardcore RPG that plops down ugly monsters in your face style.

The correct approach to "Grimdark" is replacing it with Gothic/Victorian atmosphere from vampire stories(or aristocracy(the inspiration for vampires)) and legends/mysteries/tales of medieval fantasies/tales/encounters with the unknown/evil/supernatural, not mud/blood/gore - crude means to achieve visceral feelings of dark/evil.
Being more subtle and adding elements of psychological and classic horror helps establish the setting more:

1.muted screams, whispers, shadows, ghost lights(faint and changing light sources), movements at the edge of vision, changing portraits/statues/items(when not looked at). The primary component of "spookiness" : most effective when engine allows positional audio(must-have for gothic horror). Experiment with positional audio: sounds from behind/above tend to be more effective in spooking the player. Echoes and 3D sounds amplify background effects further: ghost steps, water noises, faint music/voices, even plain white/pink noise.

2.Traps, mechanisms, automatons which respond to players movement or move on their own(as if haunted). Player shouldn't "trust their feelings", all things can be a lure/bait.
Levers, knobs, steampunk gear mechanisms, rusted machines and unreliable light systems.

3.Ghosts(as background element), Mystical light patterns(beacons, circles, shrines, bright lights from no apparent source), Lights reacting to player, Sudden loss of light, Ethereal lights, low-light sparks/fireflies, Mists/fogs/Vapors/fumes.Slime trails, Footsteps, random noises(movements/machinery).
Dynamic, multiple sources of lightning which are not controlled by player: the player has to travel through dark and light areas.

4.Mirrors(warped and normal) which show different things than reality, mirrors as portals to otherworldly locations, mirror floors/walls.Mirror corridors, mirrors in improbable places, confusing perspective from all points(if engine allows it, reflective surfaces with semi-opaque detail, semi-transparent mirrors with things inside).

5. Shadows which don't follow the player or mock him, shadows from something invisible/hidden, shadows that look like beings(with burning eyes/mouths). Things that don't
cast shadows or are always moving under it. Shadows don't have to obey optics!

6.Underground locations:crypts, tombs, catacombs, cellars, tunnels in deeply neglected/abandoned atmosphere(spiderwebs,rust, dead animals,holes(as source of danger or sound),irregular architecture/design,labyrinths), Underground springs/rivers/lakes.
Low-Light + changing light sources play great tricks on imagination:obscuring the far view is important and limited "around the corner" light sources seal the deal.

7.Water bodies at night/low light, mysterious underwater movements/monsters,floods,tainted/colored/poisoned water,etc
Fountains with strange animals inside, neglected/rusted/decaying fountains/sources of water. Insects, archachnids, worms in sources of water work well.
Leaking water, puddles, drip on walls, fungus/rot.

8.Architecture of Doom: vast vertical spaces, decay, narrow passages into wide areas,secret tunnels/rooms/passages, trapdoors, multi-level balconies/ladders/platforms.
Corrupted/Warped doors,balconies,stairs: different materials, sizes, lack of symmetry,disharmony in detail(things are not fit together and can separate/collapse/detach at any moment). Unreliable floor tiles,holes in stairs/floors/walls.

9.Creepy but not disgusting animals:Ravens(staring/sitting on objects), Rats(moving in background, in the walls, in objects, falling rats,etc), Black cats/dogs/etc in low light, Bats(colonies, sudden vertical attacks),Snakes(nests, moving from small holes), Owls(flying and screaming), Larger than usual Insects/invertebrates(nests, hives, tunnels,confined spaces).
Animal-Human hybrids/chimeras, animals with deformities, mutated humans/animals.
If crude monsters are required, predator animals with extra/abnormal/deformed limbs/eyes/appendages could make an aesthetically clear design: avoid copying humanoid features unless the monster is human.

10.The monsters: Monsters should not be "Super-animal/ugly mutant" cliche.
They need to be otherworldly, irrational/mad, shape-shifting, sentient, engaging with the player and environment(howling,screaming, taunting, crying,talking). Some may be spirits/ghosts possessing and transforming inanimate objects or other monster/npc, traveling through walls or beneath the floor. A monster doesn't have to be vulnerable all the time.
Greatest monsters are these which mimic or appear as human(Uncanny Valley).
Appearance doesn't need to horrific initially: A monster can take a disguise of little girl and transform to a multi-armed witch when player comes close.
The "monster" doesn't need to be seen, its 'presence' can create expectation of horror instead:e.g. a bulge in carpet/wallpaper moving, shadows of unseen things,etc.
The aim of "Grimdark" should be creepy atmosphere not Doom aesthetics when player rapidly shoots anything moving..