2 Chronicles 21:6
And he walked in the way of the kings of Israel, like as did the house of Ahab: for he had the daughter of Ahab to wife: and he wrought that which was evil in the eyes of the LORD. And he walked in the way of the kings of Israel, like as did the house of Ahab: for he had the daughter of Ahab to wife: and he wrought that which was evil in the eyes of the LORD.
Jehoram, king of Judah, walked in the idolatrous, evil way of the kings of Israel, like the house of Ahab. He followed that way because he had Ahab's daughter as wife. That wife was the daughter of Ahab and Jezebel, namely Athaliah.
The marriage had been consented to by his father Jehoshaphat some years after Jehoshaphat came to the throne, in the height of his prosperity. By this union an alliance was formed between the kingdoms of Judah and Israel. That alliance was not in God's order.
His evil in the eyes of the LORD included promoting Baal worship and leading Judah into sin by turning it toward Baal worship. In times of crisis that alliance later brought disaster on the king of Judah and on many of his subjects.