Albert Barnes Commentary Numbers 24:18

"And Edom shall be a possession, Seir also shall be a possession, [who were] his enemies; While Israel doeth valiantly." — Numbers 24:18 (ASV)

Seir - The older name of the mountain-land, south of Moab, and east of the Arabah, which the Edomites inhabited (Genesis 32:3; Genesis 36:8–9).