adverb taking the 'to' particle, taru-adjective
astonished; very surprised
Archaic literary word describing a state of being stunned or taken aback. Used with と or in たる form. Modern Japanese would more commonly use 愕然 (がくぜん) or びっくりした.
See also: 愕然
その知らせを聞いて、彼は駭然とした。
Upon hearing the news, he was astonished.