If I understand correctly, you, Zobr, are already working on balancing the new faction? And who are these undead in terms of lore? Are they predecessors to the undead from Heroes 6-7?

I have my own lore and world, in which:
light gnomes - colonists and traders from another continent who gained independence
knight-cultists - an army of religious fanatics who arrived from the gnomes' continent to restore the ancient human empire
undead - partially, these are priests and mages from the ancient human empire, but mostly they are knight-cultists who fell under the curse of undeath
chaos gnomes - a lost part of the gnome people, perhaps they colonized lands beyond some portal, or perhaps they simply went too deep, made a deal with the demonic lord of the ifrits.