NHF – new heroes for Heroes V 3.1.

To distance it from inhabited mines:
1. Prohibit adding creatures to the army.
2. Give it to a non-Necromancer.
3. Guard not only mines but also dwellings, maybe something else.
4. More diverse guarding.
5. Perhaps a small fee for guarding the structure.

It's starting to feel more like an RPG....
I also remember there was an idea about blowing up mines and putting them out of commission.
It's still okay for a single-player special ability.
The special ability will be passive for the hero when the enemy attacks. Also, it shouldn't be possible to disable the garrison to recruit an army, as the game already intends it for something else. For example, the hero might want to add more of their own creatures there and then take back only those that were added. And besides, there's a lot of situation tracking involved.
But if it's possible to create a kind of "spiritual connection" between the hero and the object. For example, the enemy attacks the mine, but before capturing it, they encounter our hero with the special ability.

Added 8 minutes later
I remember that in an old version of NHF, there was a hero who populated mines with Bone Throwers. It had to be removed.
There was also a hero who destroyed mines.

And I also remember a hero of the Academics who converted magical stats into physical stats