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Necromancy turns a certain percentage of destroyed units into skeletons - the math is simple there... But modifying these skeletons into other units - that's the job of the Cloak of the Undead King and only that...
Because even without the Cloak of the Undead King, if all your stacks are full and there are no plain skeletons, after the battle they will give you upgraded skeleton archers. :)
That's what I mean :) and the Cloak of the Undead King might not affect the number of upgraded skeletons, but only the number of non-upgraded ones, from which all this samba-lambada then proceeds. :)
Герои - это клёво.