taru-adjective, adverb taking the 'to' particle
glaring; blazing; fiery
Describes eyes or light that shine intensely, often with a fierce or passionate quality. Used adverbially with と or as a たる adjective in literary contexts.
彼の目は爛々と輝いていた。
His eyes were glaring brightly.
爛々たる太陽が砂漠を照らしつける。
The blazing sun beats down on the desert.
The kanji 爛 means 'to blaze' or 'to glow intensely'. The reduplicated form 爛々 emphasizes a glaring or fiery state. The exact historical derivation is uncertain, but it follows a common pattern of reduplication for emphasis in literary Japanese.