Gileraen
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Do you mean editing the campaigns of the regular Heroes (not the expansion)?
They are added via a PO mod, and the method described above can be used to edit them and put them back into the mod. If you only need to add a mentor, it will be enough to move the map.xdb file back.
And how do you split a map into files? Or vice versa?
Added 11 minutes later
I figured it out. Thanks, man. You're my hero.