=Lizard=
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It would be logical to transform them into units of the corresponding level for (for example) double the price of the transformed Necropolis unit. This would prevent the Necropolis from being overpowered. For example, a pikeman into a skeleton for 120 gold, a minotaur into a lich for 1100, and so on. This principle is well implemented in Heroes of Might and Magic V in the Haven castle, where, with unlimited resources, you can turn everyone into cavalry.