Dark champions of Necropolis.

By the way, could you give me detailed instructions on how to change a unit's stats and skills?
I would like to experiment with it myself, as I have a few ideas that I would like to test in practice.

You need to go here: data.pak/Game Mechanics/Creature/Creatures/desired castle/desired creature type;
and, as psatkha correctly pointed out, you need to know how to use an archiver (although I personally prefer a different one - WinRAR, which many dislike only because it is paid).