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She had a terrifying expression on her face at that moment.
Literal
She [topic-は] that-time terrifying face [object-を] was-doing.
~顔をしていた combines ~顔をする ('make/wear a face') with the past progressive ていた — describing a sustained facial expression at a past reference moment. 恐ろしい ('terrifying, dreadful') is a step heavier than 怖い ('scary'); it describes something that inspires real dread, not just everyday fear. The whole captures a chilling moment caught in time — narrative description of someone caught in an extreme expression.