Jamieson, Fausset and Brown on Deuteronomy 33:8-10

8-10. of Levi he said--The burden of this blessing is the appointment of the Levites to the dignified and sacred office of the priesthood (Le 10:11; De 22:8; 17:8-11), a reward for their zeal in supporting the cause of God, and their unsparing severity in chastising even their nearest and dearest relatives who had participated in the idolatry of the molten calf (Ex 32:25-28; compare Mal 2:4-6).