Pardon what I'm sure is a dumb question: If my opponent has a Hero alone in and area can adjacent units be ordered to attack it? Or is the Hero "invisible" to units when unaccompanied? If so, by the way, that would make it impossible for anyone to win the game: Horus and the Emperor just have to avoid joining any areas with units and they can't be killed.

"The order being executed instructs the attacker to choose a
target area, the one that is being attacked. A legal target
area must include at least one enemy unit or Hero." (p. 24)

FAQ also clarifies that an unsupported Hero can execute attack orders on his own (rulebook mentions that to attack you need to have a unit in the origin area).

"... at least one enemy unit or Hero" means they can be attacked. Thanks for the clarification and pointer to the rule.