I've written about symbols of HOPE before, but how about symbols of DOOM?
And the raven, never flitting, still is sitting, still is sitting,
On the pallid bust of Pallas just above my chamber door;
And his eyes have all the seeming of a demon's that is dreaming,
And the lamp-light o'er him streaming throws his shadow on the floor;
And my soul from out that shadow that lies floating on the floor,
Shall be lifted - nevermore.
(Edgar Allan Poe)
The Game of Thrones uses ravens and wolves to create tension and mystery.
In The Lord Of The Rings, Ringwraiths inspired panic and fear much like the Dementors in Harry Potter did.
But the symbol doesn't have to be an object or creature. It could use any of the five senses. In one of my novels, I use a certain sound as well as a point of light in the woods, to hopefully create a sense of foreboding.
The idea is to craft an atmosphere of intrigue, impending trouble or tragedy. It's not necessary, but certainly can ramp up the tension in your novel.