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If you're talking about this **Maps_ToE_UnlockAllHeroes.h5u** mod, it adds campaign heroes, meaning you can do the same things with them as with other heroes: choose them at the start, hire them in the tavern, etc.
In some scenarios, you can hire these heroes; in others, you can't. You need to check for compatibility and read the readme file, which is usually included with the scenario.