taru-adjective, adverb taking the 'to' particle
Literary 'taru' adjective describing a person or manner that is grand, overbearing, or full of self-importance. Often used with と (豪然と). Rare in everyday speech.
彼は豪然と会議室に入ってきた。
He strode into the meeting room with a haughty air.
その武将は豪然たる態度で敵を睨んだ。
The warlord glared at the enemy with an imposing attitude.