https://www.btstories.com/athaliah/

Jehoshaphat, king of Judah, wanted to establish a good relationship with the kingdom of Israel. To do that, he had his oldest son marry Athaliah, the daughter of Ahab, king of Israel. Unfortunately, Athaliah influenced her husband to follow the sinful ways of Israel.

As soon as Jehoshaphat died, his son became king and put idols all around Judah, and built a temple to Baal in Jerusalem. He killed his six brothers to make sure they wouldn’t take the kingdom away from him.