taru-adjective, adverb taking the 'to' particle
Literary or formal word describing intense, passionate, or vehement states, often used in written descriptions of emotions, arguments, or natural forces. Functions as a taru-adjective or adverb with と.
彼の烈々たる闘志に、誰もが圧倒された。
Everyone was overwhelmed by his fierce fighting spirit.
烈々と燃え上がる炎が、夜空を赤く染めた。
The violently blazing flames dyed the night sky red.
猛烈 is a common na-adjective meaning 'fierce' or 'intense'. 烈々 is more archaic and emphatic, often found in classical or rhetorical writing.
Reduplication of 烈 (retsu, 'fierce, violent') for emphasis. The exact historical derivation is uncertain, but the form follows a classical pattern of doubling a kanji to intensify meaning.