“King Amaziah took courage. He led out his people. He went to the Valley of Salt. He struck down ten thousand men of Seir. The people of Judah captured another ten thousand alive. He took them to the top of Sela. They threw them down from the top of so that all of them were dashed to pieces.”
The first verse is pretty much from 2 Kings, chapter 14. The second verse is an addition to the story. King Amaziah led an expedition to Edom, where he killed over 10,000 people as in 2 Kings. However, not only did they kill 10,000 Edomites, he then had another 10,000 thrown from the top of a rock at Sela so that they were smashed to pieces. Edom was southeast of Judah and had revolted against his great grandfather King Jehoram earlier. The Valley of the Salt is where King David also had defeated the Edomites.