Stone of Erech
From the One Wiki to Rule Them All, the Lord of the Rings Encyclopedia.
The Stone of Erech also known as The Black Stone was the standing stone where the king of the mountains swore an oath to Isildur. It was large, some six feet in diameter and spherical in shape and half of it was underground. The other half that was visible/above ground was approximately as tall as a man (logical problem here; if it's spherical, it would be 6 feet tall. Half of it is between 5'5 and 6'2) (If it was six feet in diameter, wouldn't that mean that it would be three feet in radius? Therefore, if half of it was buried, wouldn't it emerge from the ground to a height of three feet? Alternatively, if the Stone extended above the ground by six feet, its radius would be six feet, and its diameter twelve feet, with half of it underground, and half of it above ground). It was located on the hill of Erech.
The hill of Erech was located in Gondor, at the beginning of the Blackroot Vale in the White Mountains. The head of the Blackroot Vale was where the entrance to the Paths of the Dead was located. The stone was brought to the hill of Erech by Isildur, who rescued it from the sinking of Númenor.
In the very early days of Gondor, the king of the men who lived in the White Mountains swore an oath of allegiance to Isildur on the Stone of Erech. This oath was betrayed when Isildur called upon them to fight Sauron in the War of the Last Alliance. That caused Isildur to curse them to remain undead until they had fulfilled their oath. They haunted the Paths of the Dead for 3000+ years.
It was on the Stone of Erech that Aragorn called on the King of the dead, and the other accursed, to fulfill their oaths so that they could rest in peace. This occurred at midnight. They fought in the Battle of the Pelennor Fields (in The Return of the King (film)). In the Books, they only helped Aragorn take over the ships of Sauron's allies from the south. They were finally put to rest by Aragorn for fulfilling their oath where they had not done so before.
