The subject of this article originates from non-canonical sources. To find out about what is considered "canon" see LOTR:Canon.
The Invasion of the Barrow Downs was a conflict between the shadow armies of Angmar against the armies of Cardolan who holds the Royal Barrow against Hwaldar which he will stand atop it. If Angmar completes this mission, the Men of Cardolan will fail. The Battle began at the time of the Fall of Arnor in March TA 1409.
Cardolan wasn't affected by the destruction of Amon Sûl so the Witch-king attacks Cardolan's most sacred place of all, The Barrow Downs where their Great Kings of Arnor lie inside the Barrows, planning to draw out Cardolan's forces into a foolish and self-destructive charge. The Witch-King now needs Hwaldar to stand on top of the Royal Barrow . To assist the conquest, Hwaldar destroys the attacking forces and kills some Dúnedain rangers that planned attack the reinforcements of Angmar. With him are some Thrall Masters and Sorcerers that have gained power from the Palantír shards. When Hwaldar reaches the Royal Barrow, Cardolan becomes enraged and sends its forces to destroy him. But, the Rhudaur Spearmen & Axethrowers hold back each attack and the Sorcerers cripple and destroy Arnor's soldiers, turning some of them into corrupted Wights. As time passes, more Sorcerers, Thrall Masters, Dark Rangers, Black Númenóreans and Trolls (Snow Trolls & Hill Trolls) arrive; at one point, Morgomir joins the battle. In the end, the Prince of Cardolan himself arrives to answer the challenge, but so does the Witch-king at the same time. The Prince is eventually killed by the Witch-King and his forces, and Cardolan is laid bare to the enemy