Those of you who pre-ordered Left 4 Dead 2 (for PC on Steam or at participating retailers) now have exclusive access to the momentous event they said would never happen. For the first time in history, two separate Valve universes collide. That's right: it's a Left 4 Dead/Team Fortress crossover! This is not fan fiction! This is not a dream sequence! The characters from Team Fortress 2 will team-up with Bill's hat from Left 4 Dead 1!

And lest you think this is just some marketing gimmick, here's the intricate back-story justifying it: while conducting a science experiment on parallel dimensions, Bill's hat fell off a table into the Team Fortress universe.

Is Bill's hat the tenth class? All will be revealed in time, but don't hold your breath. Will you get to wear Bill's hat right now, in TF2? Go ahead and hold your breath—now exhale like a champ, because you're a winner and it's happening.

Even if you haven't pre-ordered yet, it's not too late to get in on this unprecedented hat-wearing, history-making crossover event! Pre-order Left 4 Dead 2 now, and the hat is yours. (Also the game at 10% off.)