NHF – new heroes for Heroes V 3.1.

Besides, this special ability is clearly not for mages, but rather for Inferno or Dungeon, for example.

Well, there were demon-worshipping mages in the "Tribes of the East" campaign; the ability was designed with them in mind. But if you don't like this approach, you can do it this way: a mage consumes an artifact in battle, and in return, he receives an N bonus to "attack" and "defense."