Jess seeks adventure at the newly opened Ephemera Cave but joins a tour led by Frank, who reveals local myths about a dangerous lower world. When Frank extinguishes the group's lights, two tourists vanish in pools of blood, and a living darkness consumes the fleeing crowd. Jess survives the initial attack but is hunted through the labyrinthine tunnels by shadowy clawed entities. Trapped as her flashlight fails, she is pulled into the underworld, where millions of creatures circle her in a grey mist.
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