What is the name of the fear demon in 'Fear, Itself'?

The fear demon in “Fear, Itself” is Gachnar.

Gachnar appears in Buffy the Vampire Slayer’s fourth-season Halloween episode after a ritual symbol is painted on the floor of an Alpha Delta fraternity house. When blood accidentally touches the mark, the demon is summoned and the party’s decorations and attractions begin turning into real threats.

Gachnar feeds on fear and traps Buffy and her friends inside the house, forcing each character to confront personal anxieties. The demon’s terrifying reputation is built up through an illustration and an ominous inscription, but the eventual reveal is one of the episode’s best jokes: Gachnar is physically tiny.

Buffy defeats him by stepping on him, making the episode’s title especially apt. Gachnar is not The Gentlemen from “Hush” or The Master, both separate Buffy villains. “Fear, Itself” aired on October 26, 1999, as season four’s fourth episode.