My idea about a soul shard fell apart due to the lore; I thought that skeletons for Necromancers were like puppets, but it turned out that skeletons can have a mind and memories of their past life... there is even a skeleton paladin killing zombies.
If the idea is cool, and in this case, it really is, then you can boldly disregard the lore...

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Неудачное планирование ведёт к неудаче ...
NCF - мод, добавляющий дополнительных существ в HeroesV 3.1