Talos is a giant made of bronze, created by Hephaestus and assigned to protect Crete from invaders. A mechanical warrior, essentially an ancient robot. He threatened the Argonauts when they approached the coasts of Crete.
Talos had a single vein within his body, extending to most of his body and containing the "blood" (fuel) giving him life. A bronze nail held it in place. If the nail was removed, the blood would spill out and Talos would die. Medea figured it out and either removed or tricked Talos himself into removing it.