taru-adjective, adverb taking the 'to' particle
blazing; fiery; burning fiercely
Describes something burning intensely and continuously, often with a dramatic or literary feel. Used as a 'taru' adjective or adverbially with と.
炎々と燃え上がる炎を見つめていた。
I was staring at the blazing flames rising up.
炎々たる太陽が砂漠を照らしつけていた。
The blazing sun beat down on the desert.
A verb meaning 'to blaze up' or 'to burn fiercely'; more common in everyday speech than the literary 炎々.
A general adjective for 'intense' or 'violent'; 炎々 specifically describes fire or heat.
Reduplication of 炎 (ほのお/えん, 'flame') with the reading えん, forming a literary adjective/adverb describing intense burning. The exact historical derivation is uncertain, but the pattern is common in classical Japanese.