Charles Ellicott Commentary 2 Chronicles 34:5

"And he burnt the bones of the priests upon their altars, and purged Judah and Jerusalem." — 2 Chronicles 34:5 (ASV)

And he burnt ... upon their altar. — See 2 Kings 23:13–14; 2 Kings 23:16; 2 Kings 23:20 for details. Literally, and bones of priests he burnt. They were bones taken from the graves of the idolatrous priests, who were thus punished, while their altars were irreparably defiled. (For the horror with which such a violation of the dead was then regarded, see Amos 2:1.)

And cleansed (i.e., purged, 2 Chronicles 34:3) Judah and Jerusalem. — This phrase does not occur at all in the parallel account.