Gentlemen

The Gentlemen are the central antagonists of the Buffy the Vampire Slayer episode, Hush and were considered one of the show's most iconic monsters, with the episode itself being praised for its genuine horror and dread in a series that tended more towards fantasy action.

History
The Gentlemen are humanoid demons who float across the ground rather than walk and are dressed in smart suits, similar to stereotypical undertakers, they were said to have originated from fairy tales and would travel from town to town seeking to harvest hearts in order to preserve their own immortality, when they arrived they would curse a town with a silence spell, preventing any noise from entering, this would be ominously foretold by the now famous rhyme:

"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."

At first Buffy is unable to find a means to combat the monsters and they begin to harvest hearts from the town of Sunnydale, until she manages to destroy a box the demons had been carrying with them, this broke their spell and she let out a loud scream, this caused the Gentlemen to literally explode, since it was close to their fairy tale weakness (the cry of a princess).

It is presumed the Gentlemen are now completely extinct or unable to return to our reality since no other members of this demon species have ever returned since.