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

I don't know of any such creatures... I rarely delve into folklore-based undead; people fear the same things – ghouls, vampires, and spirits that inflict nightmares and sleep paralysis. I don't remember anything surprising me there. And the main problem with folklore-based creatures is the lack of names for their upgrades. You could, of course, follow the principle of Heroes of Might and Magic III and add the words "kings" or "royal." But that won't work for alternate upgrades.

The word "sisters" is a great idea, meaning there's some kind of "sisterhood" – perhaps it's a coven, or maybe it's the name of a combat group. In the latter case, you could develop the story, like in ancient times, they were nuns, perhaps priestesses of the god of dreams, the god of the moon? Or maybe the sun and harvest? It's not that important; war changed everything – there are no more monasteries, no more empire, and the remaining Sorceresses hide in ruins and roam the swamps, hunting lost adventurers. Somewhere there, the Archons hang out – ancient mages and priests who sold their souls to the god of death.