taru-adjective, adverb taking the 'to' particle
Literary and rare; used almost exclusively in the fixed phrase 杳として (ようとして) meaning 'without any sign/clue' or 'completely unknown'. The standalone adjective/adverb is archaic.
See also: 杳として
彼の消息は杳として知れない。
His whereabouts are completely unknown.
杳としてその原因は闇の中だ。
The cause remains utterly obscure.