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

the outcome is already predetermined! if Dungeon starts to weaken, then even after 200 years, the Overlord will break free and destroy everything, and that outcome doesn't suit us. I still think it would be better to make demonic portals as treasure vaults and external recruitment buildings, let them be locked in Sheoh, and Sareth, as a creature connecting both of these worlds, will summon them.

Honestly, based on the video we see when we decide to free the Overlord, it can be said that the son met his father almost with open arms, and he is unlikely to set any conditions for his release, let alone overthrow him and usurp power. So, we can show at the beginning how Kha steps onto the land of Ashan with a retinue of several legions of demons, but then something suddenly happens, and the demons are forced to postpone the attack on Ashan - I think "something" can be understood as the appearance of the Order of the Void, which solves the problem of the annoying demons in a known way. The demons retreat to Sheoh, and in addition to revenge against all of Ashan, their goal will also be revenge against the Order of the Void (we have three opposing sides - demons, voidwalkers, the Alliance). In this way, we won't break the lore too much, and we will create the necessary conditions.

Added 30 minutes ago
So, at the beginning, you can play either as the Overlord, who, having broken free from his shackles, invaded Ashan with an army of demons, or as the leader of the Order of the Void, who, with his army (void spirits or whoever you have planned for the void army), attacks the demons. At the end of the scenario, the Overlord and his army of demons will be forced to retreat to Sheoh. In general, this is all probably nonsense, because the ending of Dark Messiah is not specified, and the dubious comment that we are currently relying on is extremely implausible (I'm talking about the comment that Sareth freed the Overlord and the Order of the Void appeared, etc.)