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How to get a hydreigon in pokemon black
Go to victory road.
You must first catch a deino, which can be found on the 1st and 3rd floors of victory road by walking, and it has a 20% of showing up. The location of deino is the same in both black and white.
Carry a pokemon with you that has status-affecting moves such as thunder wave or hypnosis. It would also be wise to have a pokemon who knows the move false swipe.