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Here, take a look at the calendar events for the blue player through the editor (playable red). Now, launch the game. Wait three days – if the heroes are separated, the event will be activated (the hero can also be transferred to a neutral faction so that there is no undead unit), but if the heroes are combined into an army, the event will not be activated now or later. Conclusion: you need to separate the heroes and, using scripts, change the color of the ally and make them die. Here's an idea: hire an ally not for a specific number of days, but as an escort to a specific location. But maybe someone else will come up with something.