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

What do you mean? Who is this Dark Lord? If you're talking about Kha-Belekh, then he and most of the demons were destroyed during the cataclysm. Now, there are very few demons left, but they can be summoned with a spell.

A Dark Lord could very well exist. For example, this Dark Lord could be some kind of ruler of the void, and by the way, in the Darkness creature mod, the description mentions citadels of darkness in the north, and it was for this mod that I created my first dark dwarves. I decided that the differences between them and their light counterparts are quite well explained by a pact with the Dark Lord, and by the way, Darkness includes black orcs.

Added 2 minutes later
By the way, after the cataclysm, the land of darkness could have split into undead with fallen angels and orcs with dark dwarves.