The basilisk was supposedly a fearsome reptilian creature with the power to kill human beings just with its gaze or breath. It was believed to be the product of a cockerel egg hatched by a snake or toad. It was sometimes called the king of serpents, with a regal crest on its head. First described by the Roman writer Pliny the Elder in his Natural History, it went on to figure prominently in medieval bestiaries, where it was generally depicted as being reptilian but with the head of a cockerel. The legend of the basilisk is believed to have its origins in a real-life creature: the deadly hooded Asiatic cobra.
The basilisk is a creature which, interestingly, displays features of several different traditions of legendary monsters. It is a composite beast, that is to say it combines the parts of different real-life animals - another well-known example is the griffin, a compound of eagle and lion. Its power to kill with its mere gaze recalls the more ancient, famous myth of the Gorgon Medusa, who also had serpentine characteristics, her hair being made of snakes. The serpent looms large in European tradition as a symbol of evil, and the basilisk is clearly part of this.
Although it was meant to have such lethal powers, the basilisk was not invulnerable. For one thing, it could be defeated with the use of a mirror, because if it saw its own reflection it would die.