Ment
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I suggest that the city conversion isn't immediate, but happens after a certain time limit. The higher the level, the shorter the limit. This way, a level 1 hero who follows the main hero around won't be able to use the special ability to annoy the opponent (unless the opponent takes a very long time to react), while a high-level hero will be able to (the ability to annoy the opponent is important because he doesn't have other special abilities; as Dyrman said, he needs to stand out in some way, and he won't be better than other Necropolis heroes in combat).