en_tn/oba/01/08.md

Will I not on that day"—this is Yahweh's declaration—"destroy ... mountain of Esau?

Yahweh uses this question to emphasize the certainty of the destruction of Edom. Alternate translation: "'On that day,'"—this is Yahweh's declaration—'I surely will destroy ... mountain of Esau." (See: rc://en/ta/man/jit/figs-rquestion)

the mountain of Esau

Most of the land of Esau was mountains, so this is one way of referring to the land of Edom.