A basilisk is a giant serpent, also known as the King of Serpents. In Potterverse, basilisks are bred by dark wizards by causing a chicken's egg to hatch under a toad.
Basilisks may grow up to 50 feet in length, and are colored dark green with large yellow eyes. These eyes have the power to kill anyone who looks directly into them. One who sees only the reflection of a basilisk's eyes, as in a mirror or a puddle of water, will be petrified. Ghosts, when looking straight into a basilisk's eyes, will also be petrified. Phoenixes are immune to the basilisk's gaze, and spiders are terrified of the snake.
The basilisk's fangs are full of venom powerful enough to kill a human in minutes. The venom is potent even after death. It is also one of the substances destructive enough to destroy a Horcrux. Basilisk skin is armored (rather like that of a dragon) and deflects spells cast upon it. Like normal snakes, the basilisk sheds its skin when it grows.
Basilisks can live naturally up to 900 years. Their mortal weaknesses include the crowing of a rooster and the smell of a weasel. Basilisks feed off of vertebrate animals, and the male can be distinguished from the female by a scarlet plume on its head.