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The strongest one is for Dwarves and Necropolis (without it, they are just weaklings).
For Elves, it's terrible; you have to constantly travel to the city.
For Warlocks, it's even worse; the damage increases only minimally.
For Wizards, it's too expensive! But it's cool.
For Knights, it's even more expensive and even cooler :)
For Demons, it's great, especially during sieges; it's also nice to summon them near enemy archers.