taru-adjective, adverb taking the 'to' particle
Literary taru-adjective describing something that shines or glows brightly, often used for eyes or light. Rare in modern Japanese; mostly found in classical or formal writing.
彼の目は炯然と輝いていた。
His eyes were shining brightly.
炯然たる光が闇を貫いた。
A radiant light pierced the darkness.
Common verb for 'to shine' or 'to sparkle'; 炯然 is a literary adjective/adverb describing a bright, radiant state, not an action.
Also a literary adverb/adjective meaning 'brilliantly' or 'brightly', but 煌々 is more common and often used for lamps or lights, while 炯然 often describes eyes or a piercing brightness.
Composed of 炯 (bright, shining) and the suffix 然 (indicating a state). The exact historical derivation is uncertain; it is a classical Chinese-derived literary expression.