From Team Fortress Wiki
I wish y'all could see how bad I'm kickin' your tail.
— The Engineer on compound eyes
The Buzz Killer is a headwear item for the Engineer. It is a massive fly mask with circular bug-like eyes that covers the Engineer's head entirely.
This item can only be worn around Halloween or during a Full Moon; at other times of the year, it does not appear in gameplay, unless the server has Halloween mode enabled. However, it can be equipped at any time and can still be viewed on the loadout screen even when it is not visible during games.
Engineer's vocalizations are 145% higher pitch.
October 27, 2011 Patch (Very Scary Halloween Special)
- The Buzz Killer was added to the game.
February 9, 2012 Patch
- [Undocumented] The Buzz Killer's equip region was changed to Whole Head from Hat.
- A Buzz Kill is slang for a depressing person whose presence tends to consistently darken others' moods.
- The hat is a reference to the film The Fly, based on a short story of the same name, in which a scientist accidentally merges his body with a housefly, turning himself into a monster.