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.
His eyes were glaring brightly.
The blazing sun beats down on the desert.
Kana spelling is common in modern writing, especially when the kanji are considered difficult.
Full kanji repetition spelling; less common but still encountered.
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.