Abbath
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Demons rule... against all mages.
In one turn, the demons will definitely hold out, and then, please, destroy another army like that.
The tactic is to attack with summoned creatures the first time, and with real creatures the second time, so there is no retaliation.
The Archdevil teleports the enemy to himself and finishes it off.
And why do you think a Necromancer will have a super large army? The reserves of dark energy are limited. And if it is not enough, then everyone who was transformed by the Necromancer will be lost, and there will be little point in fighting all the neutral factions.