taru-adjective, adverb taking the 'to' particle
Literary adverb describing an action or event that occurs without warning. Often used with と (to) or as a たる adjective in formal writing.
卒然と雨が降り出した。
Suddenly, it started to rain.
彼は卒然として姿を消した。
He vanished without warning.
The kanji 卒 (sudden, abrupt) and 然 (state, condition) combine to form a literary adverb. The variant 率然 uses 率 (lead, ratio) but is likely an ateji or historical variant; the exact derivation is uncertain.