What did Elisha pray would happen to the Aramean army sent to capture him?

Answer: Be struck with blindness

As the Arameans came down, Elisha prayed, and the LORD struck them with blindness according to his word.

“As the Arameans came down against him, Elisha prayed to the LORD, “Please strike these people with blindness.” So He struck them with blindness, according to the word of Elisha.”— 2 Kings 6:18 (BSB)