taru-adjective, adverb taking the 'to' particle
Literary, somewhat old-fashioned word describing a state of being frozen with fear. Often used in the adverbial form 悚然として.
See also: 慄然
その話を聞いて、彼は悚然とした。
Hearing that story, he was terrified.
悚然として立ちすくんだ。
I stood frozen in horror.
慄然 also means terrified or shuddering with fear, and is often used together with 悚然 in the phrase 悚然慄然. Both are literary and somewhat rare.
The exact derivation is uncertain; the kanji 悚 and 竦 both carry meanings related to fear or shrinking back, and 然 is a suffix forming adjectives/adverbs.