Bug: With a unit selected, units received from minor factions can be received anywhere on the map.
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Elemental Forums
Elemental Forums
I used this quite well in my first runthrough and defeat of the Sorcerer King on Beginner, Medium Magic, Oasis Map. 
Note: When the Sorcerer King knocked off one of my heroes, a trained unit was deleted from my army. There must be a way to have a defeated hero come back with an injury without knocking off a poor trained unit that was in the wrong place at the right time. 