The river in which Achilles was dipped is the Styx.
In Greek mythology, Thetis, Achilles' mother and a sea nymph, sought to make her infant son invulnerable by immersing him in the River Styx, one of the five rivers of the Underworld that grants protective powers. She held him by his heel, leaving that spot untouched by the water and thus his only vulnerable point, later known as the "Achilles' heel." This act stems from later legends, particularly from Roman sources like Statius's Achilleid , though earlier accounts in Homer's Iliad portray Achilles as a mighty warrior without explicit invulnerability.
