DocStout reminds us over at What's Next? - The Unemployed Geek that yesterday was the 14th anniversary of the date that Skynet becomes self-aware in the Terminator timeline.
I feel a little propluristemic content coming on.
What's that then? An odd concept suitable for many systems and settings, even non-gaming fiction, with the rules only a general guide, to be adapted in advance or improvised on the spot.
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The true grid
The true grid is a hyper-sophont woven through many of the fabrics of the polycosmos. While its purposes may never be known and are rarely sensed, they are largely realised. It touches minds of all kinds and through them interferes at critical junctures to manage the probabilities of future outcomes, and the courses of times and spaces are changed.
The events played out in games are presumably this kind of juncture. The interference of the true grid could be represented by a roll made on a suitable die at the start of each unit of time. If the result of the roll is a number higher than the number of the unit of time, one randomly determined entity comes under the control of another player for that period, although it may not be made to act directly against its interests.
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The key definitions are unit of time, suitable die, entity and directly.
A unit of time is a single round, turn or similar; if multiple options are available, it will be the shortest. A suitable die is a die with a number of faces equal to or just greater than the maximum number of units of time possible or likely in the game, or a die with the maximum number of faces - or dice with the greatest range - available otherwise. This could be a D6 in a game lasting five turns or a D30 - or 3D10 - in one lasting 25, maybe a D100 if open-ended. An entity is any individual, such as a trooper, character or monster, or any machine, such as a robot or vehicle, but may also be a unit if preferred. Directly may be via attacking in a wargame, or acting out of character in roleplaying._