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1) You can prevent the weekly creature spawn or monthly plague: this is achieved by connecting the HD mod plugin SOD_SP.
2) Hero leveling can be done using anything that grants experience: events, Pandora's boxes, seers, etc.
3) This can be done using the extended WOG/SOD map editor. Here is its standalone version. Extract the archive to a separate folder (not in the game installation folder!) and run it from there (it's convenient to create a separate shortcut). When saving, remember to choose the file type "Heroes 3 SOD map file": by default, saving will be in the WOG map format.
Воскресайте, камінні душі, Розчиняйте серця і чоло, Щоб не сказали Про вас грядущі: їх на землі не було…
(Василь Симоненко. Люди – прекрасні)
(Василь Симоненко. Люди – прекрасні)