The Setup:
The BLU Team lost 666 matches in a row to RED Team. They are very bummed out because of this. A "sacred number" + the color red representing hell = the end of the world as we know and
I feel fine I AM ON FIRE!!! Both RED and BLU Pyro (Arch-vile) do not want the invasion to succeed because that would kill their teams and destroy respawn. Thus they could not "play" with them. 'Play" = horrible stuff like burning them to death slowly and then resurrecting them, not that different from a typical match.
How:
BLU Pyro cuts a deal with BLU Team. BLU Team opens a portal to hell by sacrificing BLU Scout (virgin sacrifice lol) and then BLU Pyro resurrects him. This opens a portal thus starting an invasion. In return each member of BLU Team gets a
heart of hell implanted next to their uber hearts, powered by their uber blood not human souls. Using one of Hell's own weapons, BLU Team then RIPS AND TEARS the invaders, kills the Spider Mastermind behind it and become heroes of Earth. They gain fame, respect and a entire list of other stuff. RED Team inadvertently screws this up while trying to find out WTF is going on.