This is the 11533rd most frequent Korean word.
나더니
"나더니" is a combination used to indicate an action or state followed by another, often expressing contrast or transition.
'나더니' is used here to describe an action occurring first (showing up) as a lead-in context to another related follow-up action (not appearing).
그는 아침에 나더니 저녁에는 안 나타났다.
He showed up in the morning and then didn't appear in the evening.
'나더니' connects the state of the sky with the subsequent event of rain commencing.
하늘이 흐려지더니 비가 오기 시작했다.
The sky became cloudy, and then it started raining.
'나더니' serves to link the earlier action (returning home) to the following action (falling asleep shortly afterwards).