In some 4X games such as Moo2 there are random events that may occur without being caused by any human or AI player and usually have some positive or negative consequence, ranging from the loss of an empire's funds, to loss of population or industrial capacity, or even invasions by hostile alien life forms. This had the advantage of bringing up an unexpected situation and forcing the recipient player to react to something they might not have been able to foresee or prepare for. However these events were generally limited to a single star system and only affected one empire.
What about if there was a type of random event that had the potential to wipe out every life form in the galaxy?
Clearly it would have to be something more subtle than just a per turn probability of every star in the galaxy spontaneously going kaput! It would also need to be either avoidable somehow or possible to reduce the damage your empire would suffer, but in order for the event to be genuinely threatening stopping it would also have to be non-trivial. In order to justify being called a Doomsday scenario it would need to be dangerous at a scale that would threaten not just one player, but all.
A possible candidate for this kind of threat (and the only one I've thought of so far anyway =P) might be an out of control von Neumann probe. It may be more reasonable for it to be on the nanoscale, this to me is just a detail. It might be created by accident, sent as a scout from another galaxy, or perhaps found as a perverse surprise amidst the ruins of an ancient civilisation. In any case, this threat would never happen during the early game and would in fact be most likely to happen in the mid to late portion of the game, if at all. And it could be disabled from ever occurring too
The probe, or machine would head towards the nearest colonised world and attempt to 'seed' it. It would have to be either tiny, stealthy, disguised or otherwise sufficiently hidden to be able to arrive without opposition. On arrival the probe would then begin self-replicating by consuming resources. This is a way that I envisage the probe being dangerous as it would either consume minerals, perhaps take energy from the colonies' local power grid (ie.damage infrastructure), or - on a vastly more interesting and emotional note - feed off of the colonists themselves. This would at first be too small to notice, as it would be a handful of colonists out of a population that might be in billions, but the probe would multiply exponentially, and thus in time it would kill all the colonists.
Unknown threat
However, long before that happens the probe would seek to seed itself on other nearby worlds. Perhaps the probes would be capable of starlane travel independently, or maybe we can just assume they parasitically travel via a host colonist and spread as the unsuspecting host travels to another planet. Probably soon after this has happened the affected empire becomes aware of a problem with the first colony that was initially seeded, but the exact nature of the problem will not yet be apparent, it might just be reported to the player as some unknown disease or virus, though at this stage the casualties would still be fairly modest as there are still relatively few probes.
The nature of the beast
Fast forwards several turns however and the story is entirely different: the probes will have spread to several worlds (hopefully belonging to more than one empire - the probes could be biased to favour a variety of races) and by now a significant number of colonists from the first seed world have died (say a tenth) with many more falling ill on this world and others. At about this stage the owner of the first seed colony will be notified by that empire's scientists of the true nature of the threat. (And if that seems slow, then I suppose I could try to justify that by saying that the probes are small and elusive and/or designed to imitate the effects of other maladies, and if that doesn't seem reasonable just accept it for the sake of argument or else the probes would've been caught already and they wouldn't work as I want them to.)
Shades of grey goo
On learning of this threat, the player that owns the first seed colony has a number of options, one of which would include the option to quarantine the affected colonies, restrict trade (and hence civilian travel) with them and maybe close borders with any affected worlds owned by alien empires. The player may choose whether or not to announce the situation publicly to the other empires or perhaps keep it under wraps and attempt to deal with it on their own. Why might a race hold something like this back from the others? They may fear that an aggressive race might attempt to take advantage of their misfortune and begin a war whilst there concerns are elsewhere. If the player opts to stay quiet, then initiating a quarantine would probably get the attention of the other races, particularly sealing borders with the colony of another empire (with an affected planet).
Telling the truth however won't necessarily be a great deal better. A meeting of the galactic senate (if there is to be one in FO) would probably be convened and you might get some sympathy, but equally the other races will look to save their own skins and will immediately want to seal off their borders, harming trade.
Containment
Meanwhile though, security has been tightened (maybe on the same turn your scientists realise the nature of the threat, a new research option appears, allowing you to develop a basic scanner that can just about detect the probes when scanning a person, becomes available for research, or we could just say the scientists developed it the same turn they discovered the threat.) and so the probes are less able to spread by hitching a ride on a host. So the probes turn to the next stage in their development. When there are enough of them they may arrange themselves into a space flight capable machine able to traverse starlanes and spread the infection further. It is now no longer good enough to just declare a quarantine and have abstracted out border guards handle it; you need to enforce the quarantine with ships from your navy.
These probe-ships will attempt to leave orbit and spread to another system. They are small, fast and difficult to detect, but easy to destroy. Unfortunately as the infection advances down on the colony below, these ships may be produced and launched in greater numbers, plus the other worlds that got infected during the time between the initial infection and the victim player's realisation of the danger, will also begin producing the probe-ships. The player will reinforce his quarantine enforcement navy, but due to their stealthiness there is always a chance that some get through and so the infection spreads.
The bombing question
In order to try and stymie the infection, the player may use their intelligence services to try and gain leads as to where groups of infected are; sometimes the spooks will get it right and you'll be able to move against them, but not with ground forces. You may have to choose between bombarding your own colony from orbit or doing nothing to slow the infection. The population (already a bit miffed with all the deaths) will not respond well to being bombarded, and a full blown attempted exodus is under way by the remaining uninfected colonists, though many will probably be carrying probes, inadvertently doing the job of spreading the infection for them. All of them will need to be fired upon to maintain the quarantine. That and sometimes your spies will get false positives, so if you do drop bombs you'll probably kill plenty of innocent civilians, even with precision bombing, not to mention the infrastructure damage.
United they stand...
By now the infected player will probably be quite worried and may call for help in enforcing the quarantine and preventing the spread. Friends and allies may well send ships, and perhaps the galactic senate (I still forget it's real name, or if FO will have one - perhaps it might only meet in emergencies like this) may vote to send ships from many races to enforce the quarantine. If the races can throw aside their petty rivalries and unite in mutual support (and if the infection spreads sufficiently far, as I hope it would) then they will be much more successful at containing the spread, but probably not entirely as the probeships are deliberately quick and stealthy and by this point may even develop some means of attacking your ships themselves by deploying the probes against the crew, and perhaps (with sufficient technobabble justification) turning the ship from control of a regular empire to that of the probes. (Perhaps the probes can manipulate the minds of the infected?)
None so helpless as those who help only themselves...
However and this is an important idea, some empires may opt not to help you. They may judge that since there are several worlds (inhabited by an enemy empire) that they could allow the probes to weaken their rivals before fighting back against the probe. Unfortunately, by letting the probes gain access to more worlds their long-term growth rate will become dangerously high. Such an action may sound callous (and very short-sighted) but it certainly makes the game more interesting.
As for the poor victim empire that owned the original seed world, they could be able to capture one of the probes (perhaps for study, to try and find a counter-measure) or perhaps instead to use as a weapon against a rival empire. This action would become even more tempting if the victim feared they were deliberately being left to suffer.
So (assuming this event can be made to be 'winnable') I think it would be good to offer some means of preventing the initial victim from being damaged too much. Perhaps after a good while into the infection, scientists could develop a kind of vaccine that causes infected individuals to merely enter a coma instead of being killed by the reproducing probes. This would effectively set a lower floor on the population loss on each planet, based on how many colonists can be vaccinated. Perhaps instead of the initial seed empire getting this tech one of the other empires may get it instead. They could then give the vaccine tech for free, cynically charge a huge price for it or even withhold it and look the other way. In any case, once (if?) the probes were finally defeated these comatose colonists could presumably be revived, though there still would've been a significant amount of damage done to the empire by the deaths of those who could not be vaccinated.
Looking into the end of existence?
All that's left is how this will end. How can the probes be defeated? I'm going to be lazy and propose a deus ex machina. (Feel free to suggest something better!)
Eventually, it may become possible to research some galaxy-wide disruptor technology - a galactic wonder no doubt - that simultaneously shuts down and destroys all probes, with any individual/ships afflicted by them able to return to their normal state, comatose victims can be revived, etc. (And yes I know that this is stealing liberally from pop sci-fi). Or maybe the original creators programmed the probes to shut down to a special "off" signal. Maybe these solutions require certain special pre-requisite techs in combination and these pre-requisite techs are distributed amongst some of the different races, again placing a dilemma on whether the empire should be co-operative or secretive.
A second possible solution might involve obtaining special Orion technology to do the same thing. It might require a research effort at several artefacts worlds (or on Orion itself) which would then allow the galaxy-saving wonder, or perhaps there is such a device already in existence that could be utilised, or needs to be repaired with parts from other artefact worlds. Plenty more opportunities for unscrupulous races who think they may gain more than they lose by letting the infection go on a bit longer. As you can probably guess, I like such forms of intrigue, though I expect it's going to be extremely difficult to develop an AI that will play this scenario out smartly.
And if the races fail to use the deus ex machina they will eventually succumb to the power of the exponential growth curve, all remaining comatose life-forms will die as there are no longer any free-willed beings there to support them, all life in the galaxy will be wiped out and the probes, (most likely satisfied that their scouting mission is complete can go about on their way to the next galaxy and leave nothing but dust and a few more planets with ruins behind in this galaxy. This would count as the player losing the game.
Your opinions please
So what do you think? A bit game-unbalancing perhaps? Yep, I'd be inclined to agree. It will be difficult to incorporate anything like this into FreeOrion. I know I've not really provided much in the way of specifics and all of this may just have to remain a semi-dramatic story for a long-time. It's almost certainly not going to be implemented anyway, too many people would probably object to how this event wrenches the game into something completely divorced from what they expected.
But it's this game-changing nature that I think is it's strength. Even the most aggressive and angry races will have a serious stake in ensuring the galaxy is not consumed by grey goo (and if it's going to be too much bother to make an AI smart enough to realise this in general, fudge it! Write an exceptional special case script set). Bitter wars would pause under cease-fires, long-standing rivalries would have to be (grudgingly) put aside and totally independent alien empires would be likely to have to co-operate, but there is plenty of opportunity for backstabbing, shafting, cynical profiteering and even outright condemnation to near genocide.
It may seem to be too harsh on the empire whose world gets initially infected, for a random event; indeed the first world is likely to suffer huge casualties and loss of infrastructure and if the probe infection runs too far before it is destroyed, large swathes of the galaxy will be set back by years in development. However I think that with sufficient game-testing and patience, this could be made to be somewhat balanced. Not equal by any means, but then random events rarely are anyway, but sufficiently near. By letting the infected get better containment controls sooner and giving them the chance to get the vaccine, the damage they suffer can be reduced. Infected worlds won't lose all their population so long as you spend enough on getting that vaccine out, and if things really get bad you can play beggar thy neighbour and just help the probes on their way to your unhelpful rivals.
If you managed to read through all of this then thanks for your patience. Your thoughts would be appreciated
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) then anyone who finds out through spies or coincidence would be able to request a council themselves, of course in this case members of the MGB would be able to condemn the empire that kept the crisis under wraps. By the way, I forgot to mention before that other empires could still refuse the invitation to attend the convention.
ATM I'm sticking with zero point energy because that's in the tech tree. 