In the Harry Potter universe, a Basilisk is a monstrous serpentine creature. Larger than counterpart from real-life legends, this Basilisk is capable of reaching a length up to fifty feet and living up to hundreds of years. Basilisks are uncontrollable except by Parselmouths, and the first basilisk is believed to have been created by a Greek Dark wizard and Parselmouth named Herpo The Foul. Herpo made this discovery by hatching a chicken egg under a toad.

A male basilisk has a scarlet plume on its head. A basilisk kills both with its powerful venom and its huge yellow eyes, which are immediately lethal to any creature who looks at them directly. To anyone who looks at it indirectly, such as through a camera or in a reflection, it creates a profound state of petrification similar to a Medusa stare.

Ghosts who either look at it directly or indirectly will only become petrified, since they could not die a second time. A phoenix tear is the only known cure for the devastating effect of the basilisk's venom. Spiders flee from the basilisk, as they are mortal enemies. The basilisk itself flees only from the crowing of a rooster, which if heard by the basilisk is fatal, and the weasel whose odor will also kill a basilisk.

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