From Destinypedia, the Destiny wiki
Fanatics are a Vex unit found in the Vault of Glass and on Nessus. In Destiny, they are headless Goblins that glow green, and carry a debuff called the "Mark of Negation". If the Fanatic touches a player, or a player steps in a green pool left behind by a Fanatic, they will have a few seconds to cleanse themselves or else they will be killed. In Destiny 2, they appear as standard Vex enemies, where they have a modified appearance and abilities, acting as bombers that charge toward Guardians in an attempt to kill them through an explosion, while emitting a loud, glitched Vex scream. They also have a white glow, damaged appearance, and do not carry the "Mark of Negation" debuff, but can still infect Guardians.
Fanatics are glitched Vex units that seek to infect their opponents with their viscous radiolarian fluid. As they charge toward opponents, they emit loud, glitched screams from their chassis.
Fanatics serve a purpose as cannon-fodder Vex enemies, seeking to explode upon contact with enemy forces. In the Vault of Glass Raid they glow green with the Mark of Negation and attempt to infect Guardians by either exploding upon contact or leaving numerous pools of green for the unwary to step in.
List of appearances