under cover of night
Fixed expression meaning taking advantage of the darkness of night to act secretly or stealthily. Often used in literary or narrative contexts.
The enemy approached the castle under cover of night.
Composed of 夜陰 (night darkness) + に (particle) + 乗じて (te-form of 乗じる, 'to take advantage of'). The exact historical origin is uncertain, but it is a set phrase in Japanese.