In horror a film's events and images unfold outward from a structuring violation. The death of a loved one. Domestic abuse. The mounting violence and/or wrongness of a horror film is in a way an expansion and exploration of this initial wound. In Rosemary's Baby everything that befalls Rosemary, infernal and mundane, stems from her impregnation via rape. The entirety of Twin Peaks occurs within the negative space left by Laura Palmer's murder. Each new supernatural or disordering development maps the foundational trauma in greater detail, connecting it to new images and lifting it from its original context to allow for deeper examination.
In Neil Marshall's The Descent a group of friends is left directionless and distanced from each other by one member's tragic loss. After Sarah's husband and daughter die in a car accident she struggles with a sense of isolation as her friends pull away from her, unable to deal with the intensity of her grief. Sarah herself appears numb and unwilling to process the emotional ramifications of her shattered life. The group's decision to go caving together as a way to reestablish their bond functions as a metaphor for emotional exploration of Sarah's loss, and when it's revealed that one of their number has arrogantly led them into an uncharted cave system it's easy to read this development as confirmation that none of them are ready to confront the depth of Sarah's suffering.
When the carnivorous humanoid "crawlers" begin to menace the group it unleashes a wave of desperate violence which culminates in the conflation of the spelunkers and their troglodyte attackers. Their unprepared descent into Sarah's misery throws them all into emotional chaos, leading to accidental murder, the dredging up of buried secrets, and shrieking, blood-soaked brutality. When Sarah's wail of rage and desperation echoes through the caves the other members of the scattered group even mistake it for the screeching of more crawlers, unable to distinguish between their friend and the blind cave-dwelling creatures hunting her.
Every aspect of the film's mounting atmosphere of terror is designed to communicate visually the interior experience of Sarah's pain. The claustrophobic peril of the tunnels connecting the cave's galleries, the incredible physical strain of traversing great chasms, the sudden appearance of lethal holes in the fabric of the stone around the caving party. The Descent is the story of a friend group failing to pull together and it's told not in straightforward words but in blood and shifting stone and screams. It's told in broken bones and splintered nails and pale bodies flitting through the dark. It's a language only horror can teach us to speak, and to understand.