The Xiliens, who came from Planet X somewhere near Jupiter, were odd, humanoid creatures with a strangely metallic skin, although they were capable of disguising themselves to pass for human if they so chose. They were an extremely emotionless race who were vulnerable to loud noises of a high pitch.
Some amongst their number possessed the psychic ability to control any other creature whose biology included the compound known as M-base. These mutants were also gifted with superhuman strength and heightened reflexes and agility.
The Xiliens were a very technologically-advanced race, who possessed the capability of employing directed energy weapons, force fields, teleportation devices, and starships which could travel at one-tenth the speed of light.
The Xiliens have attempted to conquer Earth in two separate occasions, the first time in order to gain access to precious water, and the second to enslave the human race to allow them to harvest their mitochondrial DNA.
During their fist attempt at world domination, the Xiliens approached humanity in apparent peace and claimed that they needed to borrow the monsters Godzilla and Rodan in order to combat the depredations of King Ghidorah, to which the humans agreed. This plot was a ruse, however, and the Xiliens attempted to use Godzilla and Rodan alongside King Ghidorah to take over the world. The ploy was unsuccessful, as both monsters were able to shake off the mind control and see off the threat posed by King Ghidorah.
The Xiliens second attempt involved taking over the minds of several of Earth’s monsters and having them attack various cities across the globe, then teleporting them off and claiming to have saved humanity. Having gained the trust of humanity, the Xiliens then proceeded to replace high-ranking humans with Xilien clones, but this plot was exposed in short order, and so the Xiliens resorted to their backup plan: unleashing monsters including Gigan, Zilla, Kumonga, Kamacuras, Rodan, Anguirus, King Caesar, Ebirah, Hedorah and Monster X to conquer the planet. Godzilla proceeds to do battle with these monsters and ultimately defeats them all, whilst the Earth Defence Force deals with the Xiliens themselves.