Ezekiel 6:3
And say, Ye mountains of Israel, hear the word of the Lord GOD; Thus saith the Lord GOD to the mountains, and to the hills, to the rivers, and to the valleys; Behold, I, even I, will bring a sword upon you, and I will destroy your high places.
Ezekiel 6 belongs to the book's prophecies and earnest teaching about the moral conduct of God's people. The message was given among the captives in Babylonia. Those captives had been taken to Babylonia with King Jehoiachin in 597 B.C. The chapter falls within the first twenty chapters of the book, given while Jeremiah was prophesying in Jerusalem.
The verse means that God himself would bring a sword upon Israel and destroy its high places, so that Israel would know He is the Lord when its slain lie upon every high hill. This was fulfilled in the Roman conquest of Judea ending at Masada, where the high and fenced walls in which Israel trusted came down after siege in all its gates. The 960 slain in the high fortress show the effect of God's anger for manifold sins, and these curses remain for a sign and for a wonder.