Achilles was the son of the mortal Peleus and the nymph Thetis. A warrior of legendary prowess in battle, and the hero of Homer's Iliad, he was essential to the Greek war effort against Troy. To describe someone as Achillean is to mark that person as invincible or invulnerable, or nearly so. Achilles himself had one vulnerable spot. His mother dipped the infant Achilles in the magical waters of the river Styx in a vain attempt to render him immortal; she held him by the heel in order to submerge him in the stream, thereby leaving one spot on his body susceptible to injury. Paris took advantage of this weakness and with Apollo's help delivered the fatal arrow. An Achilles' heel refers to the one assailable feature or weakness a person may have, and in Anatomy the Achilles' tendon stretches from the heel bone to the calf muscle.