We told a bunch of people at E3, so I don't see the harm in mentioning it here:
The fifth race is Imperial Guard. It's a force localized to the planet in our story (not a planet already established by the fiction), but we use Cadians as a base. There are only a few units, and they are only in the single player game. That's all I'll say for now, so ask questions if you like, but don't expect answers.
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Jay Wilson
Lead Designer
Relic Entertainment