This idea has, in some form or another, been rattling around my skull for a while. This final incarnation owes roughly equal amounts of inspiration to Mechamorphosis, the New Doctor Who version of the Cybermen and an old Dragon magazine featuring an article on something called the "Sheen".
The basic idea is like this; strange "citadels" of a never-before-seen metal have begun to appear on the Planes. Not in great numbers, and not all that big either- generally no more than one per Plane- one per Planar Layer at most and roughly the size of a modest Prime mansion. Curious Planars, for variou reasons, try to enter the citadels- and when that fails, to contact whatever lives inside them. To no avail; the material of which they are constructed seems to be all but indestructible (though none have been seen on the Elemental/Paraelemental/Demielemental Planes as yet) and no means of communication, arcane, divine or mundane, seems to get through. Finally the Planars simply gave up and started ignoring them; it was weird that they'd shown up, but they weren't doing anything.
And then that changed...
Okay, that's the "in-game" stuff sorted out. Now for the real stuff; the citadels -for lack of a better name, I'll call them "machine cysts"- are "fragments" from an extremely alien Prime Material Plane (or perhaps The Far Realm) that have broken free of the place of their "birth" and settled upon the Planes. Perhaps controlled by some as-yet unknown entity within, perhaps sentient in their own way, the machine cysts have been "gestating"; absorbing the fundamental energy of the Plane upon which they have settled, doing so for one reason. They want to reproduce...
Now that sufficient energy has been gained, the cysts are beginning to "spawn" life-forms from within themselves, strange mechano-creatures (either Living Constructs or Aberrations- not sure which yet) emerging from their depths to stumble onto the Planes. Precisely why is a mystery yet, and maybe I'll leave it a mystery; the cysts may be "arks", and the creatures the survivors of whatever bizarre Plane from which they originated, or perhaps the cysts simply desire to conquer/destroy/assimilate all reality. Either way, the creatures are "simple" as yet; workers to gather resources and expand the cyst, observers to explore their new "homes" and warriors to defend the workers and observers. However, as time passes, more complicated and varied lifeforms will begin to "evolve", and their development is likely to be based upon which Plane the cyst actually settled in the first place.
So what do you all think? Does this sound like an idea worth developing? If so, could you please help me figure out the various details, such as the "cyst ecology" and how to actually use this idea in a Planescape campaign?
I like this idea a lot. I think it should start with the inner planes actually.
Here is my idea about the building. The building itself is alive. However, this building is alive because of a virus called the melding plague (which comes from the far realm). The virus had infected a half machine half flesh creature. This virus turns mettle into disgusting forms by combining its cells with mettle cells. In the case of the creature, the virus tries to make living and machine cells one. In actuality, the creature and the building become one, making the buiding alive (and controlled by the creature). The virus will not make things alive by itself (for example, if it affected and no machinelike creature was in it, then it would still be effected, but it wouldn't be alive), and it only effects mettle.
The virus is disgusting, it makes the building undulate with twisting "fleshy" viens, bulges, puss and ooze. At first it only effected the inside, but it covered the whole of the outside before the creatures started coming out.
I'm going to use the plane of fire as my example. The building assesses it's surroundings, absorbing matter, and adapts its creatures so they can survive there, making them immune to fire.
The first and foremost creatures that are created will be the spore launchers: These will resemble fleshy fat things, like a pod, with a hole at the top, and lots of tentacles for legs. These shoot spores out of the hole, which makes the air around them breathable for the aliens, but not for anyone else.
There will be terraformers: utterly alien in origin and look, and part of the virus, which work on the plane, terraforming it so a new building can grow, or be made. These terraformers can convert parts of the plane into different elements.
It can convert some fire atoms into earth atoms, and then can combine a fire atom with an earth atom to make a magma atom. It can then apply a positive spin on the magma atop to create an obsidian atom. This takes time. How it does this is up to you. Once it has enough of the atoms to have made a visible sample of obsidian, it passes it to the producer alien.
Producer: Producers have a large sac on there body, and they can use this to produce whatever elements they are given by the workers on a large scale. They can make 200x more of the element given to them (obsidian) and then can't make anymore until more is given, and therefore the workers need to keep giving them more obsidian. This is fast. So, while it is possible for the terraformers to create enough metal for a building, it would take many many years, whereas giving it to the producers would take considerably less time.
Next we have the builders: The builders take the infected obsidian and start to build a new building with it.
Next the Warriors: These defend the hive, or the rest of the aliens. There should be different ranks of warriors. More powerful to less powerful.
There could be small warriors, ones that attack in swarms. These could be called swarms. Make some of the warriors mimic things, like the far realms "Pseudonatural" creatures, from the template in MoTP and complete arcane. The "warrior" optiions are endless. Up to you.
The buildings themselves will be alive, so they should also be able to defend themselves. Massive tentacles? Spores shooting? Up to you, but theres my idea.