What is the name of the demon in 'Hush' that steals voices?

The demon in “Hush” that steals voices is The Gentlemen.

The Gentlemen are a group of floating, fairytale-like ghouls who descend on Sunnydale in Buffy the Vampire Slayer’s fourth-season episode “Hush.” They magically remove the townspeople’s voices, preventing anyone from screaming while the creatures and their silent footmen carve out human hearts.

The episode’s central horror comes from the loss of speech: Buffy, Willow, Xander, Giles, and the rest of Sunnydale must communicate through writing and gestures. Giles discovers that The Gentlemen can be destroyed only by a living human voice, allowing Buffy’s recovered scream to kill them.

The Gentlemen are sometimes confused with their servants, the Silent Footmen, or with other Buffy monsters. The Gentlemen are the principal villains; their minions perform much of the physical work. “Hush” originally aired on December 14, 1999, and was written and directed by Joss Whedon.