It's a long story. An ancient tribe used magic to make their city mobile by giving it giant legs, and they turned it into a weapon that would turn evil individuals into good ones. All of that negative energy built up over time, and eventually, the city could also reverse the process, turning good people evil if the person controlling it desired that. Anyone who couldn't be converted to whichever side the user wanted would be simply crushed. We managed to destroy the city, thankfully, so nobody should be able to use its power again. [She acts as though destroying a city is normal. Then again, she acted similarly when she said that she would destroy the moon, though thankfully she never intended to literally do that.]

Quite often, yes. Though generally, I've found that the more members a Dark Guild has, the weaker the individual members are, not counting the high-ranking ones. Usually, it's the smaller groups that need to be worried about.