Who did the voice in The Exorcist movie?

In the original 1973 film "The Exorcist," the voice of the demon Pazuzu, which possesses the character Regan MacNeil, was performed by actress Mercedes McCambridge. She provided the haunting voice that conveyed the terrifying presence of the demon, while actress Linda Blair portrayed Regan's physical performance. McCambridge's work contributed significantly to the film's chilling atmosphere and impact.