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The "only" makes it specific that one and only one brigade must be in play.If you have the ability without the "only," anyone with 2+ brigades would arguably have "one" in play (kinda like how all months have 28 days).
It says in play, not in territory so if you block from hand with a single color character then you have one brigade in play.
Yes, you have to have exactly one evil brigade in play. That also includes a Curse in your artifact pile, which could help our hurt you depending on the situation.