taru-adjective, adverb taking the 'to' particle
Literary, often used in written descriptions of light, beauty, or splendor. Functions as a taru-adjective (〜たる) or adverbially with と.
燦爛たる星々が夜空を飾っていた。
Brilliant stars adorned the night sky.
彼女の笑顔は燦爛と輝いていた。
Her smile shone radiantly.
Composed of 燦 (bright, brilliant) and 爛 (glowing, blazing). The exact historical derivation is uncertain, but the combination conveys intense radiance.