The voice of the Basilisk is not a physical object but a sound that traveled through the pipes of Hogwarts Castle. It is described as a cold, hissing, and venomous whisper that only a Parselmouth can understand. To non-Parselmouths, the sound is either inaudible or perceived as a faint, indistinct hissing. Harry Potter, a Parselmouth, was the only person in Hogwarts during his second year (besides the memory of Tom Riddle) who could clearly hear and understand the murderous words spoken by the beast. The voice articulated the Basilisk's primal urges, speaking of its hunger and its desire to kill, with phrases like, “…come… come to me… Let me rip you… Let me tear you… Let me kill you…” and “I smell blood… I SMELL BLOOD!”
The voice's primary magical property is its exclusivity; it serves as a method of communication and control between the Basilisk and a Parselmouth.
The voice itself is not directly harmful, but it reveals the presence and intent of the mortally dangerous creature that produces it.
The voice of the Basilisk presumably lay dormant for centuries, silent along with the creature inside the Chamber of Secrets. It was first heard in the 1942-1943 school year when a young Tom Riddle opened the Chamber and unleashed the Basilisk on the school, resulting in the death of a student, Myrtle Warren. Fifty years later, during the 1992-1993 school year, the voice was heard again after Lucius Malfoy planted Tom Riddle's Diary on Ginny Weasley. Under the diary's influence, Ginny reopened the Chamber, and the Basilisk once again roamed the pipes. Harry Potter began hearing the voice on several occasions, coinciding with the attacks on students. His ability to understand it, a secret he kept out of fear, became a source of immense internal conflict, making him question if he was connected to the Heir of Slytherin.
The voice is a central element of the plot in Harry Potter and the Chamber of Secrets.