The Team Fortress 2 Halloween event (not quite over) is a new map, Mann Manor. There are three capture points, with healing candy sprinkled about. It is a lovely piece of work with great backgrounds and details. But the new gameplay, items, and achievements are probably what you care about.
The Horseless Headless Horsemann will occasionally spawn on the capture point. He one-shots anything he can reach, and one person is “it” (melee an enemy to tag off). Kill him for an achievement and a hat, melee him for another achievement and a crafting item. Gifts will occasionally spawn randomly around the map. The first person to touch it gets an achievement and a hat. There are nine hats, a paper bag mask for each class. Get the full set and craft them together for an achievement and a hat; this is the only holiday hat you can wear after the holiday. Last year’s achievements and items are also around, and there are a few cash shop items.
