NHF – new heroes for Heroes V 3.1.

If you're going to create such a special ability, it shouldn't be a percentage-based bonus, but rather a +1 to attack, which is roughly a +5% damage increase.

Added 8 minutes later
You could either grant an army (directly in the mine, for example, to Dwarves or Necromancers), or resources (depending on the mine, the city gets gold), which should definitely depend on the hero's level. Additionally, you could exclude objects that have already been captured (or you could choose not to exclude them).