The Palantír of Amon Sûl
In harmony with the Unfinished Tales of Númenor and
Middle-earth, a Palantír (seeing stone) there was upon Amon Sûl
(in the later ages to be called, by those of Bree, Weathertop).
The palantír abiding at Amon Sûl was one of the foremost of the seven
stones made by Fëanor in Aman and brought from Númenor by Elendil and
his sons, Isildur and Anárion. The custom of the stone was mainly that
Arnor could perceive the goings on of Gondor.
The destruction of Amon Sûl in 1409 by the
Sauron-influenced realm of Angmar resulted in the Númenorean remnant's
moving the palantír to Fornost. The Lord of the Ringwraiths (Nazgûl)
was ruler of Angmar until the end of his time either by force or length
Reference: Unfinished Tales