The subject of this article originates from non-canonical sources. To find out about what is considered "canon" see LOTR:Canon.
The Witch-king of Angmar after taking the Weather Hills would have preferred to launch an attack directly on Fornost but the army of Cardolan had not been present at the battle of Amon Súl and was entrenched (near or on) the Barrow-downs. The Witch-king drew the army of Cardolan and the prince of Cardolan out into a fight by positioning troops on the barrows and slowly goading the men of Cardolan into a full-on open battle. The plan worked and the Prince rushed to retake the royal barrow but the Witch-king had only sent a small part of his army to the battle with the army of Cardolan; when they came out in the open, he sprang his trap. The army of Cardolan was massacred and the prince was slain reducing Cardolan to a few bands of refugees and Dúnedain survivors who took refuge in the Old Forest.