The Basilisk could grow up to fifty feet in length and were dark green or a color with a lighter green underbelly. They also possessed rows of incredibly sharp poisonous fangs and large yellow eyes. These eyes had the power to instantly kill anyone who looked into them.

Basilisk skin was armored like that of a dragon's, which deflected spells cast upon it. For example, Stunning Spells would not affect the deadly beast. The Basilisk shed its skin at intervals, like all other snakes, when it grew.

Basilisks could live a natural life of at least nine hundred years, though Salazar Slytherin's Basilisk lived for approximately a thousand years. This was accomplished by using Parseltongue to put the creature into a deep sleep that prevented it from aging, similar to suspended animation.

Their mortal weakness was the crowing of a rooster. Basilisks fed off vertebrate animals, but it is unknown how much they ate at one time. The Serpent of Slytherin survived on rats. The male could be distinguished from the female by a single scarlet plume on its head.

A Basilisk egg was the egg of said creature. They were chicken eggs hatched beneath a toad, thus creating the deadly King of Serpents.

When a live victim looked directly into the Basilisk's eyes, it resulted in instant death, making it even more dangerous than an acromantula. Although looking at the eyes through a camera lens or a ghost's transparent body would dampen the lethal effects, looking through a pair of glasses did not offer the same protection, because glasses still allowed one's line of vision to connect directly and clearly with the serpent's eyes.

Myrtle Warren was such an unfortunate person, as her wearing glasses did not save her from death when she looked directly into the Serpent of Slytherin's eyes.

Ghosts could look directly into the serpent's eyes without suffering death, as the dead couldn't die again; however, they would suffer Petrification. Should a camera be reflected onto the serpent's eyes, the lens and film would be melted. A phoenix was immune to the Basilisk's gaze, whether directly or not, as the bird is immortal. If the Basilisk's eyes were damaged (thus rendering it blind), it took away the lethal ability as well.