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“Shadows in the Dark”

Formless entities drifted through the recesses of Rook’s mind like mist on an impossibly dark and infinitely deep pool of water. Lightless, shapeless, and utterly cold, the horrifying tentacles pulled at him. They tugged relentlessly at his sanity, at the very core of his being. Like vampires stalking the essence of self, the shadow-thoughts hovered, preparing for the feast that would surely follow his breaking. When his control finally succumbed, when the last vestige of rebellion vanished, they would descend like vultures and tear his mind to incoherent shreds. He would be no more.

Never! The internal snarl of rage echoed in the darkness, rebounding from the unseen walls of his prison to fuel his desperate struggle. The barriers, though totally indistinct in the growing void of his tortured subconscious, were definitely closing in. Rook could sense it. He could feel a terrifying oppression like the jaws of an unsprung trap, cunningly hidden behind the seemingly undead and disturbingly foreign thoughts that assaulted his cognition. He could not hold out much longer. It was only a matter of time before his defenses failed, before the phantoms finally conquered all that he was and all that he knew.

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In the rest of the sentences, while they are not so cluttered, there seems to be a lot of repetition - "...relentlessly at his sanity, at the very core of his being." and "when his controll finally succumbed, when the last vestige of rebellion vanished" and so on.