Focusing on the atmosphere:
* As chilling as a ghost's breath.
* Like a whisper in a graveyard.
* As uncomfortable as a spiderweb in your hair.
* Like the silence before a storm.
Focusing on the feeling:
* As unsettling as a forgotten dream.
* Like a cold hand on your shoulder.
* As foreboding as a crow's call.
* Like a shiver running down your spine.
Focusing on the visual:
* As dark and mysterious as a moonless night.
* Like a fog rolling in over a haunted moor.
* As unnatural as a shadow that moves on its own.
* Like a painting that seems to stare back at you.
The best simile for you will depend on the specific context and what kind of eerie feeling you want to evoke.