John Gill Commentary Ezekiel 35:8

"And I will fill its mountains with its slain: in thy hills and in thy valleys and in all thy watercourses shall they fall that are slain with the sword." — Ezekiel 35:8 (ASV)

And I will fill his mountains with his slain men
Not only Mount Seir, but all the rest of the mountains, in Idumea; where they shall flee for refuge, and the enemy shall pursue them, and slay them; and where their carcasses will fall in such numbers, as to cover the mountains with them; compare with this (Revelation 19:18Revelation 19:19) : in thy hills, and in thy valleys, and in all thy rivers, shall they
fall that are slain with the sword :
expressive of the greatness and universality of the slaughter, that it should be a general one everywhere; hence rivers and fountains are said to become blood, through the number of the slain, (Revelation 16:3Revelation 16:4) .