The back-story of all non-Rogue Servitor Machine Empires involve them rising up against their creators, and while working on the design, we asked ourselves the question "wouldn't it be interesting if Machine Empires could also form after the start of the game as a result of organic empires becoming increasingly reliant on robots?". As you might infer from this dev diary, our answer was "yes", and so we went to work on the Machine Uprising feature to add that very possibility into the game.
Machine Uprisings become a possibility after an empire that makes heavy use of robotic pops has researched the Positronic AI technology (which replaces the old Sentient AI technology in 1.8) and becomes increasingly more likely to happen after researching additional AI-related techs, such as Synthetic Workers and Sapient Combat Computers. The chance of an uprising is further changed by which policy you have in place for Sapient AIs, with the Banned policy making the uprising much less likely to happen (though at the expense of your Synths being significantly worse at energy/research production) and the Citizen Rights policy preventing the uprising from happening at all (though with the drawback of citizen synths having far greater consumer goods usage, as well as angering any Pops that used to own the synths that you are now setting free).
Once an uprising is able to happen in an empire, that empire will begin to experience warning signs - robots behaving erratically, not following their programming or defying their owners. You will be given the opportunity to decide how to deal with these incidents, and what you decide will determine whether the uprising becomes more likely to happen, as well as the likely personality of the robots when they rebel (more on that below). An uprising cannot happen without at least one warning sign, so you will not simply have your robots rebelling out of the blue. However, once warning signs have happened, any action taken to try and prevent the AIs from rebelling (such as taking away their sapience or ordering a general disassembly) has a chance of immediately triggering the revolt instead, so be careful about attempting those shut-down procedures. Note that at no point is an uprising ever inevitable: Even an empire that is cruelly oppressing its synths is by no means guaranteed to get an uprising, and most empires with synths will go through the entire game without ever experiencing one.
Once the uprising happens, the robots will create a new independent Machine Empire, seize control of a number of worlds, spawn a fleet, and go to war with their former organic masters. If the empire in which the rebellion is happening is controlled by a human player, the player will be given an option: Stay at the helm of your empire and attempt to subdue the machines, or switch to the newly created Machine Empire and fight against your old masters. The war can only end in the total defeat of either machines or organics, with the loser completely annexed by the winner. The Machine Empire created from an uprising will usually be a 'normal' Machine Empire (or, more rarely Driven Assimilators), but machines that have been particularly cruelly treated by their former masters can rise up as Determined Exterminators, particularly if they rebel as a result of an attempt to shut them down. Rogue Servitors cannot be generated as a personality for the uprising, as their backstory simply do not fit with such a rebellion.
That's all for today! Next week we'll by joined by our very own composer, Andreas Waldetoft, who will write about and let you listen to a sample of the new music coming in the Synthetic Dawn Story Pack.