Albert Barnes Commentary 2 Chronicles 21:11

"Moreover he made high places in the mountains of Judah, and made the inhabitants of Jerusalem to play the harlot, and led Judah astray." — 2 Chronicles 21:11 (ASV)

See 2 Kings 8:18. The writer of Kings only tells us in general terms that Jehoram did evil in the sight of the Lord and walked in the way of the house of Ahab.

Here, in 2 Chronicles 21:11, 13, we have the particulars of his idolatry. Jehoram, it seems, was seduced by the evil influence of his wife—Athaliah, the daughter of Ahab—and permitted the introduction of Baal-worship, with its idolatrous altars in various high places, groves (Asherahs), images, and pillars. The people were not only allowed but were compelled to take part in these new rites. The phrase to commit fornication is a common metaphor for idolatry or spiritual unfaithfulness (compare 2 Kings 9:22).