A swarm of nanites flood the area and cause everything outdoors to degrade (roof does not help)... Colonists will die within 2 hours, having their limbs slowly disintegrated.
Should only last between 3 days and a week...
Visual effect could just be little grey particles flowing over the map. Could be made even more threatening by allowing the nanites to chew through wooden walls...
Can I ask what purpose this would bring other than a completely BS way of killing off your colony? What would be the gains of rebuilding your entire infrastructure if your colonists did survive? Please help me understand your thought process with this.
It's just an idea anyway... I was thinking that the entire point of the game is to survive disasters long enough to build a space craft to get off the planet... at a later stage, something that could cause extinction seems like a valid addition to the game... It's avoidable and can obviously be tweaked...
The nanites would only disolve organic matter, so trees flesh and crops would be eaten... Building walls of stone or steel would save you, and colonists would have to stay indoors like in a toxic fallout... The time period could be taken down to 1 or 2 days if it seems too harsh...
seems pretty cool to me
I think the idea has potential, but a week would be too extreme. Instead id suggest a wave of locusts type deal, where the landscape gets a single wave of it, similar to how fire behaves.
Get a warning "mechanite swarm approaching from the north / South / east / west!" and several minutes to prepare. Once it arrives, it covers every non wall tile gradually, filling it up... Spreading until it hits the other end of the map / consumes everything. However, walls stop it (while taking say, 40 damage) as does everything else outside, taking damage from tiny cuts all over. So it would kill / maim everything outdoors, but only last 24 hours, possibly more if a huge map.
This way it will be swift, harsh, yet still survivable, and wouldn't have a ridiculous long effect like toxic fallout.