From Destinypedia, the Destiny wiki
Fanatics are a Vex unit found in the Vault of Glass and on Vex machine worlds such as Nessus. In Destiny, they are headless Goblins that glow green, and carry a debuff called the "Mark of Negation". In Destiny 2, they appear as standard Vex enemies, where they have a modified appearance and abilities.
In their first appearance in the Vault of Glass Raid, Fanatics resemble standard Goblins who have been decapitated, and will glow and sparkle green. Upon being destroyed, they will leave behind a green pool. Should a player step into this pool or come into contact with the Fanatic itself, they will be afflicted with the "Mark of Negation" debuff, and only have a few seconds until they are killed by the Templar's Ritual of Negation.
In Destiny 2, Fanatics will slowly advance toward the player while firing on them with their Slap Rifle and emitting glitched screams. Once they get too close (In which case they briefly glow blue) or are damaged enough, they will explode, leaving behind a pool of Radiolaria which damages those who step into it. If killed using a Finisher, they will be launched backwards with a delayed detonation, and will not leave a Radiolarian pool behind.
List of appearances