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Yes, playing Heroes "blindly" without a computer played a trick on me, and as a result, I unintentionally misled...
Here is a post from Sir Catch, the Rector of the school of basic mastery for Heroes II:
"Nuances of Vampire Lord regeneration in Heroes II and III.
As it recently turned out during a discussion on a friendly resource, even experienced Heroes players do not know the differences in Vampire Lord regeneration between II and III. Moreover, after studying the regeneration mechanics in one version, they extrapolate them to the other, which can mislead less experienced players.
So:
1. Heroes II. Vampire Lords regenerate from absolutely all monsters, including undead, elementals, golems, and gargoyles.
Heroes III. Vampire Lords regenerate only from living monsters; they do not regenerate from the aforementioned ones.
But almost everyone knows this point.
2. Heroes II. Vampire Lords regenerate only from killed units.
Heroes III. Vampire Lords regenerate from wounded units as well.
3. Heroes II. Vampire Lords regenerate only in full; that is, if they kill monsters totaling less than 40 HP per turn, they do not restore their numbers.
Heroes III. Vampire Lords can regenerate partially. That is, even when dealing damage equal to one HP, one unit of our vampire stack will be restored (provided all others were fully healthy at the moment of the strike).
Now let me explain with examples.
We have a stack of vampires (here and hereafter, upgraded ones, meaning Vampire Lords) and a stack of goblins. Suppose the vampires fly in and attack. The goblins, in turn, strike (80) and take down two vampires. The vampires strike back and deal 50 damage. In II, one vampire is restored, but fully. In III, two vampires are restored, but the last one will be "wounded," having 10 health.
Next. We have a stack of vampires and a stack of Black Dragons. The Black Dragons strike and destroy 3 vampires (120). The vampires strike back and deal 50 damage. In II, not a single vampire will be restored (since there are no killed units). In III, two vampires will be restored (one fully, the second with 10 HP). But! When, in the course of the battle, the vampires hit a mortally wounded Black Dragon—for example, the Black Dragon has 20 HP left and the vampires finish it off—then: in II, a Vampire Lord will restore 300 (!) HP, meaning 7 (or 6, if the last unit in the stack had more than 20 damage) bloodsuckers will rise from hell, whereas in III, only 20 HP will be restored from what remained of the Black Dragon. And that might not even be enough to restore a single vampire.
Knowledge of these nuances often wins battles fought with vampires. For example, we have a stack of tanky, high-HP monsters (Ogre Lords, Hydras, etc.). We estimate how much damage our vampires will deal and realize we are just a bit short of killing a unit. Then we remove some HP using a faster monster (gargoyle, lich, etc.) or via magic, for example, from a shooter. Or we cast Bloodlust on the vamps to increase their damage. In the end, the monster is killed, and the vamps are restored. This works especially well against particularly tanky monsters, from which several vampires, rather than just one, can be restored. In III, however, one should generally attack monsters with the lowest defense so that the vampires can gain the maximum amount of health (although every battle has its peculiarities; it might be worth blocking shooters or, conversely, finishing off a powerful monster that can deal substantial damage to the bloodsuckers).
And for both versions, it is true that defensive spells are very effective for vampires (Stone Skin for both versions, Steel Skin (2), Shield (3)), as well as those that increase the attack level (Bloodlust, Dragon Slayer (2), Slayer (3)). Also, Destructive Ray and Curse on enemies."
Thus, all calculations regarding the confrontation between the Black Dragon and Vampire Lords were erroneous; I will either delete or correct my posts on this matter shortly. By the way, in connection with the above, the first-level spell "Bloodlust" looks particularly attractive for a Barbarian with Vampire Lords, as it lasts three turns regardless of magic power.