The forest at night was quiet, and only the sound of the wind could be heard.
彼は密やかに部屋を抜け出した。
He quietly slipped out of the room.
二人は密やかな恋を続けていた。
The two continued their secret love affair.
Standard kanji spelling.
Kana spelling is common when the kanji is not needed or for softer nuance.
The word is composed of 密 (secret, dense) and the adjectival suffix やか. The exact historical derivation is uncertain, but the kanji reflects the core meaning of secrecy and quietness.