Final answer:
The horror contorted the character's face in fear, indicating an overwhelming sense of dread.
Step-by-step explanation:
The effect of the horror on the character's face caused his face to contort in fear. The phrases describing the horror as 'twisting' and his face 'darkened with some powerful emotion' suggest a strong physical and emotional response, not calmness or anger. The correct answer is that the horror made his face contort in fear, indicating that the character experienced a significant and perhaps overwhelming sense of dread or terror. Descriptions of facial expressions and emotions are common devices used in literature to convey a character's inner turmoil and the impact of certain events or revelations on their psyche.