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

I quickly reviewed the history of Erathia. Belket is essentially the father of necromancers. Erathia is a passive state (with the exception of the incident with Markel and the War of the Broken Staff); it's unlikely that the necromancers will all unite and decide to start a war. For that, they need a determined leader like Arantir.
As for the plot, we should first decide on the ending of Dark Messiah so that everything is clear later.
I didn't understand about the Order of the Void; what is this order, anyway? And will there be otherworldly interference from the Void in the plot of RTF, in the form of dark angels with an army of the Void, or will the Altgarde simply be the troops of the order?

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I don't think this is a good idea.

Excellent, so everything is much simpler.