Xilien

The Xiliens are a sapient species of cybernetic reptilian humanoids who long ago interbred with humanity, causing their species to gain the M-base. They can alter DNA in order to add M-base to other species, which they have the ability to control. They have used not only Earth's own daikaiju against it, but also three extraterrestrial daikaiju: King Ghidorah, Gigan, and Monster X. They naturally have the ability to become Keizers.

Physiology
Xiliens evolved from a reptilian species on Planet X. By the time they first came into modern contact with Earth, they were a completely emotionless race, letting their lives be run by computers, with their leader known as the Controller. Everything during this first era was numbered, likely because of their computers; to this effect, King Ghidorah, Godzilla, Rodan were known by the Xiliens as Monster Zero, Monster Zero-One, and Monster Zero-Two respectively.

Xiliens once interbred with humanity long ago, which added the DNA compound known as M-base to their genetic structuring. Because of this, some humans were born as mutants, many of which would be destined to become soldiers of M-Force. An extremely rare type of mutant known as a Keizer are occasionally born with enhanced "super powers" due to the fusion of M-base and Human DNA. Keizers are able to control any other lifeform by telepathic means so long as that organism possesses M-base.

They are vulnerable to loud, high-pitched sounds.

Technologies
Initially, the Xiliens were in possession of spacecraft that are capable of one-tenth the speed of light, which utilize explosive laser beams fired from domes on their undersides. These ships can also encase targets as gargantuan as kaiju inside of giant force-bubbles, which are then transported using tractor beams to haul them great distances - at least as far as the distance between Earth and Planet X. During their second invasion, however, they had a much larger vessel - the mothership - which had three attachable capitol ships and hundreds of small fighter craft. The capitol ships were capable of "beaming up" daikaiju, which they likely transferred into the mothership for holding purposes. The mothership is immense and spherical, with three claw-shaped capitol ships capable of linking to its lower hemisphere. The mothership is protected by a powerful forcefield, whose generator is located in the core of the ship itself.

In terms of handheld weaponry, they had developed laser pistols that could disintegrate a human being in a single prolonged shot.

Their greatest technological prowess however was the ability to take control over any creature's mind, so long as M-base is present.

The First Invasion
The intelligent inhabitants of the mysterious Planet X, the Xiliens invaded Earth twice. On their first attempt, they pleaded for assistance from Earth's populace, stating that King Ghidorah was now assaulting their homeworld, only a year after he had been defeated by the combined forces of Godzilla, Rodan, and Mothra. They asked mankind if they could be allowed to "borrow" both Godzilla and Rodan, and were then allowed to transport the daikaiju to their planet, where the two succeeded in driving the three-headed dragon away.

Shortly after this, however, the Xiliens seemed to have a change of heart, and stated that they now had complete control over Godzilla, Rodan, and King Ghidorah. With this statement out in the open, they then gave the people of Earth twenty-four hours to choose between surrendering themselves over to Planet X or to be destroyed by the monsters. Eventually tiring of awaiting an answer from the Earthlings, they unleashed all three kaiju anyways upon Japan.

Finally, it was revealed that water was very scarce on Planet X, and was therefore highly coveted by the Xiliens, who also needed it to survive. This was their main reason behind their invasion of Earth. At the end of the first invasion, the Xiliens were defeated when the Earthlings discovered their weakness to high-pitched sounds. At some point after this attack, they were all but forgotten by the populace of the world (although whether this was due to a cover up or through some other means is not well understood).

The Second Invasion
Forgotten by their second attempt, they were able to covertly take control of many of the Earth's kaiju by altering their DNA remotely to include M-base, which they then could use to control them. With this, they then dropped each of their pawns into various cities in secret, confusing and frightening the populace of the world. Rodan was sent to New York City, Anguirus to Shanghai, a Kamacuras to Paris, Zilla to Sydney, King Caesar to Okinawa, Kumonga to Arizona, and Ebirah to Tokai. Easily proving to much for the EDF to handle at once, they successfully caused rampant destruction - however, once enough damage was done, the Xiliens sent in the capitol ships and teleported the monsters out of each city, much to the surprise of Earth.

They announced their existence by appearing over the Earth Defense Force headquarters, and proceeded to mask their true intentions with peaceful purposes. They stated that they had rescued the people of Earth, and that they wanted only peace between their two planets. Next, they warned humanity of a small, run-away planet known as Gorath that was on a collision course with Earth, and that they must aim all of their firepower at it or be destroyed.

In truth it turned out that Gorath didn't actually exist, and was probably used either as a way to get Earth to trust the Xiliens or for Earth to expend all of its ammunition to prevent them from being able to launch a revolution at a later date against the Xiliens, if not both. Originally planning to use Xilien clones to aid them in slowing giving power over to the inhabitants of Planet X, but they were soon discovered when it was noticed that they never blinked. When confronted with this and exposed on national television, the second-in-command - who had always believed in their superior firepower and technology as opposed to bureaucratic manipulation - shot and killed their leader, taking command of the invasion force from that point on, becoming the new Controller. He then explained that the reason for this invasion was to use mankind as a sort of food source - the Xiliens required mankind's mitochondria to survive. He then returned to the Mothership, and released the kaiju once more to terrorize Earth, this time assisted by swarms of Xilien fighters. As an extra measure, he activated his people's ancient weapon - a mummified, cyborg kaiju named Gigan.

Despite Godzilla having been controlled by the Xiliens only 39 years earlier, he seemed to be unfamiliar with the creature, and when M-Force used the Gotengo to awaken the frozen dinosaur from his icy tomb, he sent all of the monsters under his control to defeat him. One by one, Gigan, Zilla, Kumonga, and Kamacuras all fell, followed by a team attack by Rodan, Anguirus, and King Caesar, who also failed.

With Ebirah destroyed next alongside Hedorah (who appeared to just be in the wrong place at the wrong time), the Controller became infuriated. With no more creatures to fight with, he sent his secret, last monster to battle: Monster X, contained inside of an enormous asteroid. Monster X was much more powerful than the other monsters Godzilla had fought before, and overpowered the lizard numerous times. Unfortunately, the Controller was also unfamiliar with Mothra, and was thus unprepared when the creature came to Godzilla's aid, eventually leading to her kamikaze into a repaired and upgraded Gigan.

As the last two kaiju fought one another, the Controller went to battle with a human Keizer named Ozaki, which ended in the Controller's defeat. Completely insane by this point, he caused the ship to self-destruct, leading to the entire fleet's defeat due to the explosion.

In the case of their last monster, it too was a Keizer, and transformed into another Ghidorah.

Appearances

 * Invasion of Astro-Monster (1965)
 * Godzilla: Final Wars (2004)

Mentioned, Non-Extraterrestrial Diakaiju

 * Anguirus
 * Ebirah
 * Kamacuras
 * King Caesar
 * Kumonga
 * Mothra
 * Rodan
 * Zilla

Trivia

 * The Xiliens are often called Xians by American fans, though this is not an actual name.