Emphasizing ethereal nature:
* Like a wisp of smoke
* Like a shadow cast by moonlight
* Like a memory fading from view
* Like a whisper on the wind
* Like a dream slipping away
Emphasizing presence/absence:
* Like a chilling breath on your neck
* Like a cold hand touching your shoulder
* Like a presence in the room you can't quite see
* Like a flickering candle in the dark
* Like a voice calling your name from the shadows
Emphasizing sadness/loneliness:
* Like a lost soul wandering the earth
* Like a broken mirror reflecting nothing
* Like a bird with clipped wings unable to fly
* Like a forgotten photograph, faded and fragile
* Like a single tear falling from an empty eye
Emphasizing fear/horror:
* Like a nightmare come to life
* Like a creature from a dark fairytale
* Like a scream swallowed by the night
* Like a chill that seeps into your bones
* Like a sudden gust of ice-cold wind
The best simile to use depends on the specific tone and context you want to convey.