expression, suru verb (precursor)
be reduced to ashes; be destroyed by fire
Literary expression meaning complete destruction by fire. Often used in formal or dramatic contexts, such as describing buildings or cities burning down.
The castle was reduced to ashes by the war.
The entire town was destroyed by the great fire.
全焼する means 'completely burn down' and is used in everyday news reports, whereas 灰燼に帰す carries a more formal, classical tone.
From classical Chinese, combining 灰燼 (ashes) and 帰す (return to). The phrase is used in formal Japanese to describe utter destruction by fire.