AstralLein
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Generally, it seems like the seventh unit for Necropolis is vampires, which are carefully hidden. And the bone dragon is a separate unit. But this is not certain. It's based on hints.
And no, there are no thieves in the game; this is a definite fact. And there shouldn't be any, as they don't originate from Jadama. The identity of the sixth faction is unknown. As are the "forest elves," because there are no elves there. The only elves in the game are dark elves. Although the developers created some confusion with the foresters, which led to the belief that the Sylvan faction in Olden Era are supposedly forest elves.