AstralLein
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And not just against the undead. Also against golems and elementals.
Added 1 minute ago
How about this special ability:
If a hero's knowledge increases, their spell power increases by the same amount.
Perfect for an Academy hero.
Isn't that a bit too overpowered? When developing a hero, you need to carefully manage these skills; otherwise, the hero will be almost useless. But here, it's simple: give a mage hero education, and they're good to go.