He desecrated Topheth,1 which was in the Valley of Ben Hinnom,2 so no one could use it to sacrifice his son3 or daughter in the fire to Molech.
References for 2 Kings 23:10
He removed from the entrance to the temple of the LORD the horses that the kings of Judah4 had dedicated to the sun. They were in the court near the room of an official named Nathan-Melech. Josiah then burned the chariots dedicated to the sun.5
References for 2 Kings 23:11
He pulled down6 the altars the kings of Judah had erected on the roof7 near the upper room of Ahaz, and the altars Manasseh had built in the two courts8 of the temple of the LORD. He removed them from there, smashed them to pieces and threw the rubble into the Kidron Valley.9
References for 2 Kings 23:12
The king also desecrated the high places that were east of Jerusalem on the south of the Hill of Corruption--the ones Solomon10 king of Israel had built for Ashtoreth the vile goddess of the Sidonians, for Chemosh the vile god of Moab, and for Molech the detestable11 god of the people of Ammon.12
References for 2 Kings 23:13
Josiah smashed13 the sacred stones and cut down the Asherah poles and covered the sites with human bones.
References for 2 Kings 23:14
Even the altar14 at Bethel, the high place made by Jeroboam15 son of Nebat, who had caused Israel to sin--even that altar and high place he demolished. He burned the high place and ground it to powder, and burned the Asherah pole also.
References for 2 Kings 23:15
Then Josiah16 looked around, and when he saw the tombs that were there on the hillside, he had the bones removed from them and burned on the altar to defile it, in accordance17 with the word of the LORD proclaimed by the man of God who foretold these things.
References for 2 Kings 23:16
The king asked, "What is that tombstone I see?" The men of the city said, "It marks the tomb of the man of God who came from Judah and pronounced against the altar of Bethel the very things you have done to it."
"Leave it alone," he said. "Don't let anyone disturb his bones18." So they spared his bones and those of the prophet19 who had come from Samaria.
References for 2 Kings 23:18
Just as he had done at Bethel, Josiah removed and defiled all the shrines at the high places that the kings of Israel had built in the towns of Samaria that had provoked the LORD to anger
Josiah slaughtered20 all the priests of those high places on the altars and burned human bones21 on them. Then he went back to Jerusalem.
References for 2 Kings 23:20