also: かっかく
taru-adjective, adverb taking the 'to' particle
Literary or classical adjective/adverb describing something shining brilliantly or being glorious. Often used in written descriptions of light, flames, or splendid achievements. The 'taru' adjective form is archaic and mostly found in older texts; modern usage is rare.
赫々たる太陽が砂漠を照らしていた。
The brilliant sun shone down on the desert.
彼の功績は赫々たるものだ。
His achievements are glorious.