What specific threat does Agamemnon make to Achilles?

Agamemnon threatens to take Achilles' concubine, Briseis, as punishment for losing his own prize, Chryseis. He asserts that he will seize Briseis in front of Achilles, thereby humiliating him and demonstrating his authority as the commander. This act is meant to assert Agamemnon's dominance over Achilles and is a pivotal moment that escalates their conflict in "The Iliad."