The Gentlemen are a group of hideous and truly terrifying monsters that appeared in "Hush" - an episode of Buffy the Vampire Slayer, despite this being their only appearance they have become an icon of sorts for the show as one of the more sinister and memorable monsters to terrorize Sunnydale - the episode is also fairly unique in most of it have little to no dialogue due to the evil curse the monsters put upon the town, which rendered all living people mute.
The Gentlemen are said to have their origins in fairy tales and travel from town to town harvesting organs, hearts to be exact, in order to keep themselves alive but suffer from one fatal weakness: the sound of human scream is deadly to them and in order to combat this they utilize a magical-box to render all living beings silent in the towns they visit, malevolent specters their silence does little to hide the menace they pose and a disturbing nursery rhyme is formed (which Buffy hears in a dream):
" Can't even shout, can't even cry
- The Gentlemen are coming by.
- Looking in windows, knocking on doors,
- They need to take seven and they might take yours.
- Can't call to mom, can't say a word,
- You're gonna die screaming but you won't be heard."
Indeed shortly after this dream the Gentlemen do arrive in Sunnydale and begin their gruesome harvest - Buffy and her friends are powerless at first to stop the murderous fiends as the curse has rendered them all mute but Buffy manages to eventually destroy the box during a battle with the Gentlemen and lets out a loud, long scream - which causes the Gentlemen to literally explode, ending their threat.