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

Sareth did free the demons, but they didn't conquer the world; they became, let's say, "aggressive neighbors" to the other factions.

I believe that such an outcome is only possible without a master. For example, Sareth and Zana decided to lock away the master, but demons can be summoned in many other ways:
1) Opening a gate
2) Summoning, like Fenrig for Zana
3) Rune gates in Grimheim
4) A permanent portal, as in the Arantir campaign (where a demon appeared once a week)
5) The Griffin's Heart
Besides, it's unlikely that the developers in Heroes 6 would have revealed Zana's image for no reason, showing that she is relatively good.