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Did you know this:
Let two Heroes, each with 1 unit in their army, meet. Now, press "Split" and transfer 1 unit from one Hero to the other. As a result, the Hero will have no units but will have 2000 MP (by Xarfax111).
You can get a Hero without units in various ways, but using this... I don't know if it's a feature or a bug... because moving without monsters isn't really intended... I need to think about it!! Seriously!