"The Hill and Stone at the head of the Blackroot Vale"1,
in the Lamendon province of Gondor.
Save for Tolkien's writings in Unfinished Tales, English
language writings expanding on the above seem to be scant (I found
the finnish definition "Alkuperäinen teksti. Erech
Kukkula Gondorin länsipuolella jossa Isildurin Kivi oli",
but that didn't help.
Actually, it is suffice to say that Erech was merely a hill and
an ancient stone, presumably "fallen from heaven"2. What is more interesting
is the dark history of the Oath of Erech, merely alluded to
in the Lord of the Rings, and expounded upon in
The stone, a black sphere ten feet in diameter was set upon the hill in 3320 S.A. by Isildur
as a symbol of the royalty of Gondor and her kinship with ancient Númenor.
The men of the White Mountains (the Druédain) were required to
swear fealty to the Dunédain upon it. However, "after the
return of Sauron when Isildur summoned the men from the
mountains to fulfil their oath and join the last Alliance
S.A.3430 they would not come because they had in the dark years
before worshipped Sauron."4
Enraged, Isildur laid a curse upon the aboriginal woses*,
declaring that they would "never find peace and rest until their
oath was fulfilled"5, naming Erech (for their King of
the Drughu was named the same as the hill) as the last
King. "The men of the mountains did not dare to go to war upon
either side, they hid themselves in the mountains and slowly
In the Third Age Aragorn II "Elessar" summoned the Men of
the White Mountains to the Stone of Erech, where they in turn
fulfilled their oath, when Ghân-buri-Ghân "guided the
Rohirrim down the Stonewain Valley on their way to Minas
Tirith"7 and the Battle of the Pelennor Fields in
March 3019 T.A. (In reward Aragorn gave him and the Druédain
the Druadan Forest for perpetuity, forbidding any man to enter
it without their leave.)
In the "real world", Erech--also known as Ancient Uruk--
"length", or "Moon-town", was one of the cities of Nimrod's
kingdom in the plain of Shinar (Gen. 10:10)
8, one of the first cities established after the
Biblical account of the Great Flood.
* The Men of the Druadan Forest were known by many names:
in the Sindarin of Gondor "Druédain"; in their own
tongue, "Drughu" (pl. of Drûg); and in common
speech, "Púkel Men", or "woses".
1 from The Encyclopaedia of Arda, http://www.glyphweb.com/arda/e/erech.html
2 ibidem. Though the Danish site "Annals of Arda" says it "was set there by
Isildur at his landing, brought from Numenor after the Downfall."
3 Tolkien, J.R.R., Unfinished Tales, 1982 Unwin/Hyman ed., pp. 377-387.
4 at http://www.annalsofarda.dk/annals-of-arda/Places-index/Places-ME/Erechs-Stone.htm
5, 6 ibidem
7 Unfinished Tales, p. 382
8 from The Bible Encyclopaedia, http://www.christiananswers.net/dictionary/erech.html