psatkha
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or maybe make Void Spirits a powerful unit like Liches (use a calculator, Psatkhа, to help you) since they won't join in every battle. The unit's name could be Shadow Lord (of the Void) or something related to his resurrection (he was killed in Mythgard).
The point is that outside the NCF, the number of creatures is limited!
Sincerely, Psatkha