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

reasoning about demons is, of course, good, but then they need to be introduced as a separate race or...
or someone turned to them for help (it turns out to be Sareth from the Alliance of Light, he might have needed the help of demons (however silly it sounds) against the Order of Void (a campaign for which is simply necessary))

Added 1 minute ago
and then, through scripts and summoning neutrals, a campaign for Sareth can be made, but how does this relate to the magical blade and Lethos?

let's do it this way: Sareth, after killing Aranthir, teleports somewhere to create an army of demons and we'll leave the old storylines, but without him, and after the destruction of the Unicorn Empire and the formation of the alliance of three races (humans, elves, dwarves), he will appear in Grimheim near the Gates of Shio, and there will be a super battle, after which all existing factions will be seriously weakened and will not have time to deal with the surviving demons, and Sareth with his followers will occupy the dungeons under the ruins of Talongard and organize his capital there

before the super battle, the balance of power will be as follows:
1) the alliance of Lethos and the Ice King (Chimera mages, Dark Dwarves, Ice Nagas of Shanria, neutral mercenaries from the free cities, and the Black Guard, who switched to Stonehelm after Aranthir's death)
2) the remnants of the old world (elves, humans, dwarves in a single castle, elementalists, possibly nagas)
3) voidlings
4) proud loners
- Stronghold (with the arrival of gnolls, the fragmented clans will unite under the leadership of a strong leader and will be able to compete in strength with any race)
- orcs (the army of the Ice King will unite into an alliance only after the destruction of the orcs (a common enemy will bring anyone closer), after which the orcs will hide in the Dragon's Tooth Mountains, and the desert orcs of Sahaar, on the contrary, will create a powerful state)

as a result, the demons will cease to pose a serious threat for about 200 years and will give way to our races
it turned out well:D