What follows is a campaign idea worked out for a fantasy campaign, and it uses monsters found in that setting, but, with a little work, you could use it for almost any role playing system short of sci-fi ultratech.
The malevolent race known as the Illithid (Mind Flayers) can well remember when their world Ith was a glorious paradise, when it's black jungles bloomed wild, when the warm, primordial oceans washed the unspoiled beaches, when the Shrouded sunned themselves atop armorlichen spires miles high, their razor sharp foreclaws glittering the violet light. All the Illathid race was happy, and lived at peace, with just the right amount of inter-racial violence to ensure nobody would get bored.
That was long ago, and now strange things are beginning to happen to the Mind-Flayer garden of Eden. The sun of their homeworld is becoming brighter and stronger. The seas are getting warmer, the foliage greener, and in time, the Illathid will be driven underground. Their home planet will soon no longer support them. Several centuries ago, the grand and glorious Order of the Ebon Tentacle finally seized the initiative from it's competitors and gathered almost a thousand Illathid in the great chamber of council. Heads bowed in reverence, the multitudes breathlessly awaited the commands of the great mind. At last the brain pool spoke, and it ordered it's children to cast themselves upon the Astral seas, searching for another world, a place where the ecology of Ith could be recreated down to the minutest detail, and the Illithid could once again live in the relative peace of constant interracial war. And so it was done, and not long ago, some fortunate Illathid began a hive on a world known to it's inhabitants as "Earth". The planet's measurements seemed exact, slightly larger than Ith even, with a comparable atmosphere and seas that had the potential to support Ith life with some modification. But the sun there was too bright for Illathid existence, forcing the first pioneers to take shelter underground. It was then that they found the moon. It was worshiped as a god by the local inhabitants, but the Illithid knew better. The moons of Ith were smaller by far, and completely unsuitable for the purposes that the Illithid had in mind.
Tunnelling out the centre of the moon, the Illithid plan to create a perfectly circular chamber that is in effect a giant lifejammer helm. The life force of thousands of living creatures will be necessary to accomplish the goal; move the moon to a position in which it will block the sun's light, and hold it there. Luckily, the planet beneath is crawling with living creatures of no particular consequence that will make excellent sacrifices... The Shrouded would just eat them anyhow, once the ecology of Ith is transplanted. And so was launched possibly the most half-baked plot for world domination ever cooked up by sentient creatures that smell faintly of squid and eat human brains.