expression
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.
敵は夜陰に乗じて城に近づいた。
The enemy approached the castle under cover of night.
Similar meaning, but uses 闇 (darkness) instead of 夜陰 (night darkness). 夜陰に乗じて is more specifically about night, while 闇に乗じて can refer to any darkness.
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.