taru-adjective, adverb taking the 'to' particle
Literary and somewhat archaic; describes a dignified, imposing, or stern manner. Often used adverbially with と (げんこと) or as a たる adjective in formal writing.
He attended the meeting with a grave demeanor.
The judge solemnly read out the verdict.
The exact derivation is uncertain; the kanji 儼 (solemn, dignified) and 乎 (an archaic auxiliary character) combine to form a literary expression for gravity or sternness.