It is said that the mounds of Tyrn Gorthad, as the Barrow-downs were called of old, are very ancient, and that many were built in the days of the old world of the First Age by the forefathers of the Edain, before they crossed the Blue Mountains into Beleriand, of which Lindon is all that now remains.
–The Lord of the Rings, Appendix A
The Barrow-downs or Tyrn Gorthad are a set of downs south-east of the Old Forest. The location of an ancient burial ground from the Elder Days, these hills were part of the Kingdom of Arnor and later a stronghold and burial site of the Kingdom of Cardolan. After that realm's destruction by plague and war, the Witch-king of Angmar sent terrible spirits to haunt the barrows, and there they still dwell. This is a gloomy place, prone to sudden fogs when its inhabitants approach the player, but a brave treasure-seeker can find many riches in the old ruins and barrows - if they have the courage to face the perils within. The Barrow-downs are bordered by the Old Forest to the West and by Eriador on all other sides.
Upon entering the Barrow-downs, you will get the achievement "The Hills of the Dead".
For some reason, the Barrow-downs contain all the normal animals, including wild boars instead of pigs, even though the animals were said to have been driven out by the Barrow-wights. Even birds and butterflies spawn.
Barrow-Wight - Deadly and mysterious spirits that wander the hills at night (and some even linger during the day). Because of fear of these wights, few enter these hills, and fewer still return alive. When the wights spawn near the player they cause a dense fog to envelop the landscape...
The Barrow-downs contain a few sparse trees that can be oak, spruce, birch or, most commonly, dead variants. There is an increased amount of double tall grass here, which once could also grow inside the barrows. Flowers grow little in these desolate hills.