1 Grond. I always get shivers down my spine when I read how it advaces towards Minas Tirith. I remember I first read
The Siege of Gondor at night and then I had to sleep with my light on
2. The trees of the Old Forest attacking the Hedge. Actually make that the trees of the Old Forest in general.
3. The "dark things" that "creep from sunless woods" and "houseless hills" Aragorn talks of at the council of Elrond. I know they're probably only orcs or trolls, but the way he speaks about them scares me.
4. The man found in the Paths of the Dead lying beside a closed door, with his fingerbones still clawing at the cracks. That's really a terrifying image.
That's about all I can think of for now that has not been said before...