As with Renegade Ops, Valve is letting a third party take advantage of its IP to promote the Steam release. Steam pre-orders of Dungeon Defenders will net you some exclusive Valve-related bonus downloadable content, most notably the Aperture Science Handheld Portal Device for use in-game. In addition, you can get some Team Fortress 2-themed familiars.
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Given the four-player, cooperative, and tower-defense nature of Dungeon Defenders, it's easy to imagine how a Portal gun might come in handy, which you can see in action in the trailer above; however, the Team Fortress 2 familiars that appear in the trailer probably bear a little more explanation.
Familiars are basically helpful pets that give players extra battlefield support. The Team Fortress 2 familiars that are part of the pre-order deal include the Heavy, Medic, Engineer, and Pyro. The Heavy adds some extra ranged firepower with his Gatling gun; the Medic can heal a group of allies; the Engineer can repair nearby structures, and the Pyro burns foes to cinders.
Dungeon Defenders will be available on on October 19.