Albert Barnes Commentary 2 Chronicles 20:15-17

"and he said, Hearken ye, all Judah, and ye inhabitants of Jerusalem, and thou king Jehoshaphat: Thus saith Jehovah unto you, Fear not ye, neither be dismayed by reason of this great multitude; for the battle is not yours, but God`s. To-morrow go ye down against them: behold, they come up by the ascent of Ziz; and ye shall find them at the end of the valley, before the wilderness of Jeruel. Ye shall not need to fight in this [battle]: set yourselves, stand ye still, and see the salvation of Jehovah with you, O Judah and Jerusalem; fear not, nor be dismayed: to-morrow go out against them: for Jehovah is with you." — 2 Chronicles 20:15-17 (ASV)

In 2 Chronicles 20:16, the prophet uses words that were familiar to the people, connecting the event to several of their great past deliverances.

The “cliff of Ziz” (or more accurately, the “ascent of Ziz”) refers to the mountain path leading up from Engedi. This path crosses the elevated area still known as El-Husasah and heads in the direction of Tekoa (2 Chronicles 20:20).

The phrase At the end of the brook should be understood as “at the end of the gully,” or dry riverbed. To this day, no name like Jeruel has been found in this district.