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

Well yeah... in 7 these liches are essentially nazgûls, in 6 the mounted knights, and in 5 the old ones... but they need to be called something like lesser liches, although you could also leave them as is! Dragons into neutrals...

for 5 I'll move the flying undead (which use the angel model from seven), for 6 the Knight, for 4 the shooters — guys with shovels (don't remember if I sent them or not), 3rd and 1st are ready, just need to sort out the second

I think that the undead can dwell in cursed lands where the dead cannot find peace; skeletons wander the land, their bones held together by shackled souls. These very souls sometimes overcome the curse and regain consciousness, and then join brotherhoods of liches, undead, or knights. Now, they overcome the curse through anger, devotion to something, or intellect — hence the 3 guilds. There is also a fourth guild — Necromancer, these are beings capable of generating dark power and raising the dead outside the cursed lands; for the survival of the Factions, this is a very important guild, which is why the others follow them.