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Sooooo... In order to prevent her son from turning to a life of evil, Rose opted to leave the Overlad in Nordberg. This is knowing that he had glowing eyes of doom and lightning shooting out of his fingers and that the town people were not likely to take kindly to someone so different from them. Is anyone surprised he went from being a picked on child to a full-fledged evil Overlord?
The fact that she spent the next few decades helping the Empire dedicated to persecuting people with magic either makes this worse, or maybe indicates she just didn't care about the kid.
She left her child in a backwater place with people that possess less brainpower than the Minions, and far away from most of the rest of civilization. She then marries Marius and (supposedly) becomes Leader of the Sentinels. This means she's on top of everything they find, and can try to steer them away from Nordberg for as long as possible. Remember, it took the Minions a few years to find the Overlad, as well.