Discussion of third-tier monsters.

The Vampire is the best at level 3, but considering the growth and so on, it's generally difficult to determine...

Oh... Actually, no. Play 5 Crusaders against 3 Vampires. Just have them engage in close combat. You'll see how your favorite Vampires fall... ;)

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What's so good about the Vampire? Is it that it can restore health, or is it that it can dominate with the help of Necromancy?

It's good for both. But it's the appearance, and it's deceptive.

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Nightmare.
Because it's strong, and because it has Terror (oh, how many times it saved me)

It's a pity that Terror can only be cast once, but the problem is that if there is magic resistance, then Terror may not work:D