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I agree that they are key figures who can bring victory. And if an opponent underestimates them, they may pay dearly for it. Hence, the eternal problem of the Necromancer with Vampires. Should they be thrown into battle immediately? Or hidden in their ranks, and when the enemy's troops lose numbers, unleash them like a hedgehog (you can also add Divine Strength, Stone Skin, Vampirism, etc.) and engage in a fierce battle.
P.S. No matter what anyone says about Vampire Lords, I believe they are stronger than regular ones (my IMHO). Because Stun can take a strong enemy stack out of the game for 2 turns, and possibly even 2 stacks. Yes, there is a chance that it won't work. But as they say, he who does not risk does not drink... you know what xDDDD