Judean king Manasseh ruled from 698-642 BC, beginning his reign when he was only 12 years old. Migrants from the destroyed Northern Kingdom had brought their Canaanite characteristics into the Southern Kingdom, despite Hezekiah's reforms. Canaanite cults flourished, including astrology and Ba'al worship (Ba'al is a traditional Canaanite deity). Also, the name Manasseh itself is from the Northern Kingdom, which is unusual considering Hezekiah's Adonai-worship. Manesseh grew desperate as Edomites and Assyria exerted pressure on Judah, even going so far as to perform the Phoenician practice of sacrificial infanticide.
1 Manasseh was twelve years old when he began to reign; and he reigned fifty-five years in Jerusalem: and his mother’s name was Hephzibah. 2 He did that which was evil in the sight of Adonai, after the abominations of the nations whom Adonai cast out before the children of Israel. 3 For he built again the bamoth [high places] which Hezekiah his father had destroyed; and he reared up altars for Baal, and made an Asherah [Asherah pole], as did Ahab king of Israel, and worshiped all the army of the sky, and served them. 4 He built altars in the house of Adonai, of which Adonai said, “I will put my name in Jerusalem.” 5 He built altars for all the army of the sky in the two courts of the house of Adonai. 6 He made his son to pass through the fire, and practiced sorcery, and used enchantments, and dealt with those who had familiar spirits, and with wizards: he worked much evil in the sight of Adonai, to provoke him to anger. 2 Kings 21:1-6