The Haunted Hallowe'en Special Title update for Team Fortress 2 has 5 achievements.
Collect 20 Halloween pumpkins from dead players to unlock a hat.
Kill a player scared by a ghost.
Cause the deaths of 5 players by exploding nearby pumpkin bombs.
Kill a Spy disguised as your current class.
Dominate a player wearing the Ghastly Gibus to earn your own.