RTF is a global modification that replaces all towns in the game.

But at the same time, making the entire 3rd column a "void" column is not interesting. Plus, then it would turn out that the previous 5 levels should also be "void," which, in my opinion, is a bad idea. Plus, how do you make, say, an air elemental related to the void? Then we get an air-void elemental, an earth-void elemental, and so on. Or how? :D

It's not necessary for all units to be related to the void. You just need to choose approximately the same color, for example:
Alt-grade:
1) Battle Cabiri (you can get away with just painting the armor)
2) Storm Elementals (because they are darker)
3) Mist Elementals (because they are darker)
4) Flame Elementals (energy elementals, blue, will go to the Simurghs and Djinns in the 2nd column)
5) Onyx Elementals (because they are darker)
6) Void Djinns (because they are darker)
7) Void Eagles (or something like that, but you'll have to give up on the Simurghs)
Upgrade
1) Master Cabiri (you can decorate them by painting the armor)
2) Some kind of Hurricane Elementals (make them blue/light blue)
3) Ice Elementals (because they are lighter)
4) Energy Elementals (because they are blue)
5) Some kind of Lapis Lazuli Elementals
6) Medium Djinns
7) Simurghs
Something like that.