Sworn enemy.

Shadar, you're going to buy the Tome of Darkness, and I already presented an argument for it! It costs 20,000, and if you want to rush early, you'll have to do without it! Besides, there's also the question of whether you'll have sworn enemies!

Where am I going to find it for you, and besides, I'm not relying on the Tome of Darkness and not trying to find it... again, the magic mirror, and the Academy troops have a lot of immunity! And don't worry about sworn enemies... everything will be fine.
P.S.: A sworn enemy is also good against those units that the enemy hero specializes in.