Which actress provided the rasping unnerving voice of the demon inside Linda Blair in the "The Exorcist"?

Mercedes McCambridge (b. Carlotta Mercedes Agnes McCambridge in March 1916 – d. March 2004 in La Jolla, California) was an American actress of radio, stage, film, and TV. The famed director Orson Welles called her "the world's greatest living radio actress." She won an Academy Award for Best Supporting Actress for her screen debut in "All the King's Men" (1949) and was nominated in the same category for "Giant" (1956).

She was known for providing the rasping unnerving voice of the demon Pazuzu in "The Exorcist" (1973). This supernatural horror film concerns the demonic possession of a young girl and her mother's attempt to rescue her through an exorcism conducted by a pair of Catholic priests.

Pazuzu is a fictional character who is the main antagonist in "The Exorcist". He was created by William Peter Blatty. Blatty derived the character from Assyrian and Babylonian mythology, where the mythic Pazuzu was considered the king of the demons of the wind, and the son of the god Hanbi. In "The Exorcist", Pazuzu appears as a demon who possesses Regan MacNeil played by Linda Blair.