Losing the battle, what did the king of Moab offer as a burnt offering on his city wall?

Answer: His firstborn son

King Mesha of Moab sacrificed his firstborn son, heir to his throne, on the wall. Great fury came against Israel, and they withdrew home.

“So he took his firstborn son, who was to succeed him, and offered him as a burnt offering on the city wall. And there was great fury against the Israelites, so they withdrew and returned to their own land.”— 2 Kings 3:27 (BSB)