What did the LORD throw down on the Amorites fleeing from Gibeon, killing more than the sword?

Answer: Large hailstones

As the Amorites fled, the LORD cast down large hailstones, and more died from the hail than from Israel's swords.

“As they fled before Israel along the descent from Beth-horon to Azekah, the LORD cast down on them large hailstones from the sky, and more of them were killed by the hailstones than by the swords of the Israelites.”— Joshua 10:11 (BSB)