As its punny name suggests, a Scream Fortress map is a Team Fortress 2 map designed to be extra spooky, with additions like zombies, arcane magic and new, thematically-appropriate achievements. Er, excuse me, according to Valve that should read "Necromannchievements." In practice, new Scream Fortress maps are a usually just a change of scenery for most regular Team Fortress 2 players, but any excuse to gun down magical wizard ghosts is a good one.
There are no caveats attached to these five re-released Scream Fortress maps. Anyone with a copy of Team Fortress 2 can jump right into the Halloween gunplay after a relatively quick, free download.