noun, taru-adjective, adverb taking the 'to' particle
sorrowfully; with a heavy heart
Archaic/literary word describing a deeply sorrowful or melancholy state. Often used in classical or highly formal written contexts. The 'taru' adjective and 'to' adverb forms are typical of classical Japanese grammar.
Sorrowfully, he left his hometown behind.
With a sorrowful expression, she was gazing out the window.
Compound of 愁 (しゅう, 'grief, sorrow') and 然 (ぜん, a suffix indicating state or manner, from classical Chinese). The exact historical derivation is uncertain, but it follows a common pattern for forming literary adjectives/adverbs in classical Japanese.