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.
Everyone was overwhelmed by his fierce fighting spirit.
The violently blazing flames dyed the night sky red.
Variant spelling without the iteration mark; less common but equally valid.
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.