The earliest mention of the basilisk comes from Nicander of Colophon, a Greek poet and physician who lived in the second century BCE. In his didactic poem the Theriaca, a treatise on venomous creatures and their antidotes, Nicander described the basiliskos as the most poisonous of all serpents, a creature so deadly that it earned the title "little king" because all other snakes fled before it. Unlike the later medieval monster, Nicander's basilisk was primarily a creature of extreme toxicity, a small serpent whose venom overwhelmed its victims with rapid fatality. It was not yet the gaze-killing horror of later legend, but the seed of that terror was already planted in its regal name and unmatched deadliness.

The basilisk received its most famous classical description from Pliny the Elder, the Roman naturalist who compiled his Natural History around 77 CE. Pliny placed the creature in Cyrenaica, a province of North Africa in what is now Libya, and described it as no longer than twelve inches, about the length of a man's hand to his elbow. Its body bore a bright white marking on its head like a sort of diadem, the crown that gave it its royal name. Unlike other snakes that advanced in manifold coils, the basilisk moved with its middle raised high, sparing the earth beneath it as if too proud to crawl. Its breath scorched grass and withered bushes before it even touched them. Its touch burst rocks. Birds flying overhead were burned and devoured by its mere presence. Pliny recorded a chilling account of a man on horseback who killed a basilisk with a spear, only to have the venom rise through the weapon and kill both rider and horse, proving that the creature's poison needed no bite to kill.

Yet even this king of serpents had its match. Pliny declared that the venom of weasels was fatal to the basilisk, a fixed decree of nature that nothing should be without its remedy. Men would throw basilisks into weasels' holes, which were easily known by the foulness of the ground around them. The weasels would kill the basilisks by their stench, but would themselves die in the process, and nature's battle was accomplished. This pairing of the basilisk and the weasel became one of the most enduring images of the creature, a symbol that the deadliest evil always carried within it the seed of its own destruction. The weasel, armed with the herb rue and its own natural ferocity, became the basilisk's necessary counterweight in the medieval imagination.

The basilisk transformed dramatically during the medieval period, growing from a small venomous serpent into a hybrid monster of nightmare. Isidore of Seville, the seventh-century archbishop and encyclopedist, described the basilisk in his Etymologiae as a creature whose name meant "little king" in Greek, the ruler of all serpents, whose odor killed other snakes and whose gaze killed men. He noted that no flying bird could pass unharmed by the basilisk's face, but however distant it might be, it was burnt up and devoured. Isidore also recorded that the basilisk could be overcome by weasels, and that people took weasels into the caves where basilisks lay hidden. But it was in the medieval bestiaries of the twelfth and thirteenth centuries that the basilisk achieved its most terrifying form, depicted as a cock with a serpent's tail, hatched from a rooster's egg by a toad or serpent, a creature that combined the pride of a king with the corruption of unnatural birth. Medieval Christians saw in the basilisk the very image of Satan, the king of demons, and the weasel that slew it as a symbol of Christ's triumph over evil, armed with the herb rue that represented divine grace.

The basilisk's legend persisted into the Renaissance and beyond, appearing in heraldry as the symbol of Basel, Switzerland, where it served as a canting supporter alluding to the city's name, with records tracing such usage to coins from 1384. The creature appeared in the arms and architecture of Europe as a guardian of terrible power, its image carved into stone to warn away evil. Even as natural historians like Conrad Gesner began to question the basilisk's existence in the sixteenth century, the creature's power over the imagination never faded. It became the ancestor of every gaze-killing monster that followed, from the cockatrice of English folklore to the serpent of J.K. Rowling's Chamber of Secrets, a creature whose deadly stare still terrifies readers two thousand years after a Greek physician first wrote its name. The basilisk endures as the eternal symbol of invisible, unavoidable death, the little king whose crown is a warning and whose gaze is the end of all things.