Sworn enemy.

Generally, the ability is good, but it's hampered by the fact that we can only change our blood kin in our own castle. But what if it's on the other side of the map? As the game progresses and you clear out areas, the list of blood kin becomes a mishmash of the most annoying and unpleasant monsters in your opinion, and by the endgame, it would be nice to be able to tailor the list to your opponent, but traveling to the castle is very risky. I have a suggestion: why not add a building to the map, or add an extension to existing buildings (like the dwarven rune repository, the mage repository, the well, or the military academy, etc.) that would give you the ability to change your blood kin once the building is built. And even better, right in the hero's window. I don't think this would significantly change the balance.

Added 4 minutes later
Other races use their ability without being tied to a city, except for mages, who can only craft artifacts in the city. This could also be adjusted, for example, in a machine workshop.