The priests were to represent God to the people. This is exactly what Phinehas did on this occasion. He executed God"s wrath against sin and punished the sinners. In so doing he atoned for the sin by representing Israel before God, and he restored the covenant. God rewarded him by promising that his descendants would enjoy peace and would occupy the office of the high priest forever (cf. Psalm 106:30-31). This they did (cf. Judges 20:28) with the exception of a short interruption in Eli"s days. The Romans finally broke up the Israelite priesthood. [source][source][source]
This everlasting covenant of peace guaranteed a privileged position of service to God to Phinehas and his descendants. It will find final fulfillment when the descendants of Phinehas, through Zadok"s branch of Phinehas" family, serve God in the millennial system of worship by offering memorial sacrifices in the temple. [1][source]