He desecrated Topheth, which was in the Valley of Ben Hinnom, so no one could use it to sacrifice their son or daughter in the fire to Molek.
References for 2 Kings 23:10
He removed from the entrance to the temple of the LORD the horses that the kings of Judah had dedicated to the sun. They were in the court near the room of an official named Nathan-Melek. Josiah then burned the chariots dedicated to the sun.
References for 2 Kings 23:11
He pulled down the altars the kings of Judah had erected on the roof near the upper room of Ahaz, and the altars Manasseh had built in the two courts of the temple of the LORD. He removed them from there, smashed them to pieces and threw the rubble into the Kidron Valley.
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 Solomon king of Israel had built for Ashtoreth the vile goddess of the Sidonians, for Chemosh the vile god of Moab, and for Molek the detestable god of the people of Ammon.
References for 2 Kings 23:13
Josiah smashed the sacred stones and cut down the Asherah poles and covered the sites with human bones.
References for 2 Kings 23:14
Even the altar at Bethel, the high place made by Jeroboam 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 Josiah 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 accordance 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 people 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 bones.” So they spared his bones and those of the prophet who had come from Samaria.
References for 2 Kings 23:18
Just as he had done at Bethel, Josiah removed all the shrines at the high places that the kings of Israel had built in the towns of Samaria and that had aroused the LORD’s anger.
Josiah slaughtered all the priests of those high places on the altars and burned human bones on them. Then he went back to Jerusalem.
References for 2 Kings 23:20