taru-adjective, adverb taking the 'to' particle
terrified; horrified; trembling with fear
Formal or literary term describing a state of being deeply frightened, often with physical trembling. Used in written language or dramatic narration.
その光景に慄然とした。
I was horrified by that sight.
慄然として立ちすくんだ。
I stood frozen, trembling with fear.
Compound of 慄 (ritsu, 'fear, trembling') and 然 (zen, a suffix forming adjectives/adverbs indicating state). The exact historical derivation is uncertain, but the word is used in classical and literary Japanese.