Splatt
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The issue has already been discussed. It was suggested that a vampire drains energy, including the energy used by a Necromancer to raise undead. And it was fixed a long time ago.
But they had to be weakened, otherwise there would be an imbalance.
Added after 10 hours 12 minutes
And in general, vampires in Heroes of Might and Magic IV had a small growth rate. But in Heroes of Might and Magic V, they have a large growth rate...