Numbers 25:18
For they vex you with their wiles, wherewith they have beguiled you in the matter of Peor, and in the matter of Cozbi, the daughter of a prince of Midian, their sister, which was slain in the day of the plague for Peor’s sake. For they vex you with their wiles, wherewith they have beguiled you in the matter of Peor, and in the matter of Cozbi, the daughter of a prince of Midian, their sister, which was slain in the day of the plague for Peor’s sake.
The verse describes how the Midianites harassed Israel with crafty deception that tricked them into breaking God's commandments, including seduction into Baal-peor worship. That deception was Balaam's counsel to Balak, given to secure reward and promotion after Balaam found he could not curse Israel directly because Israel's prosperity depended on keeping God's law. Balaam told Balak to proclaim an idolatrous feast in honor of idol gods, to draw the Israelites with music, and to employ the most beautiful Midianite women to entice them to corrupt themselves through sexual immorality and to offer sacrifice to idols. That snare, laid at Balaam's suggestion, brought destruction on many thousands.
Cozbi named in the verse was the sister of the Midianites and the daughter of a Midianite prince, who was put to death in the plague that came because of the Peor apostasy. Because the Midianites had harassed Israel in this way and led the people into sin, God commanded Moses and Israel to harass and strike the Midianites in return. That command to oppose Midian came through Moses as the word of Christ, Israel's unseen leader, who has power both to create and to destroy. The command was given after the Midianites had filled up the measure of their wrongdoing, when their probationary time had ended and the door of mercy was closed to them. In its setting the verse belongs to the Baal-peor account that shows the difference between disobeying God and clinging to Him, for the followers of Baal-peor were destroyed while those who stayed faithful remained alive.