Here are my suggestions for adjusting the narrative voice to better suit a suspenseful horror story for adults:
Increase the sense of dread and foreboding:
Amp up the visceral fear responses:
Make the basement more threatening:
Create more uncertainty and paranoia:
Foreshadow the horrors to come:
The oppressively dark hallway closed in around Sarah as she crept forward, cold sweat prickling her skin. She reached a trembling hand out to the basement door, which creaked open with a sound like a dying breath. Peering down into the abyss, a sour taste of dread filled her mouth.
She descended the rickety wooden stairs, each step groaning under her weight as the darkness swallowed her. Ghostly cobwebs brushed her face and she shuddered, icy fingers of fear trailing down her spine. The dank, rotting smell of the depths filled her nostrils.
At the bottom, her flashlight flickered erratically, as if fighting to stay alive. Shifting shadows danced at the edges of the wavering beam. Sarah's muscles tensed, breath coming short and fast. The suffocating silence pressed in on her, broken only by skittering sounds just outside the light.
Steeling herself, Sarah took a step forward, determined to uncover the house's unspeakable secrets. But deep down, a rising sense of doom told her she was descending into the belly of the beast, where nightmare fuel awaited her in the inky black depths.
The suggested changes aim to:
The revised excerpt amps up the suspense and horror elements to better target an adult audience while preserving the core story beats and character motivations. The ominous tone and mounting sense of doom are intensified to hook readers and foreshadow the terrors to come.