Sworn enemy.

A sworn enemy deserves respect! But it's good that it works 50/50, and in quick games, it's less effective.

It's not always exactly 50/50; you can influence the outcome with education, and with Sylvan's cunning, it can be 60/40, especially in combination with other castles. And it seems to me that, indeed, when playing as Haven, the attack on sworn enemies triggers more often than with other heroes.