Weathertop, known as Amon Sûl in Sindarin, is where the Nazgûl attack Aragorn and the four Hobbits, and Frodo is stabbed in the shoulder by a Morgul-knife. Weathertop is the southern most peak of the Weather Hills, and sits at the intersection of the realms of Arthedain, Cardolan, and Rhudaur.
"On the flat under-side Frodo saw some scratches: 'There seems to be a stroke, a dot, and three more strokes,' he said.
'The stroke on the left might be a G-rune with thin branches,' said Strider. 'It might be a sign left by Gandalf, though one cannot be sure. The scratches are fine, and they certainly look fresh."
—Excerpt from chapter "A Knife in the Dark" from The Lord of the Rings