Monster X

Monster X (モンスターX, Monsata Ekkusu) is a skeletal monster from Godzilla: Final Wars, serving the Xiliens. His attacks include gravity beams from his eyes, melee attacks, and transforming into the more powerful Keizer Ghidorah. He is also shown briefly to levitate for a short period of time. Even before transforming, he's still extremely powerful. He's a physical match for Godzilla and his gravity eye beams posses roughly the same strength as Godzilla's thermonuclear breath. He's also the only monster Godzilla fought in Godzilla: Final Wars that survived getting blasted by Godzilla's thermonuclear breath at point-blank in the face (actually appearing to shrug the attack off), as well as being the only one able to bring Godzilla to his knees, and make him scream out in pain, with just one attack.

Monster X
Monster X was summoned by the Xilians as their trump card against Godzilla, coming to earth inside the Gorath asteroid. Godzilla and Monster X commenced battle, with Monster X holding an initial advantage, and then fought to a standstill. Mothra arrived to even the odds, and in response, the Xilians sent out the upgraded Gigan, who quickly dispatched her, and then assisted Monster X. Monster X and Gigan were briefly able to overcome Godzilla, when Mothra attacked once more and knocked both of them down.

Gigan resumed his battle with Mothra, and both were killed. Although Monster X had the advantage Godzilla soon seemed to gain the upper hand as he knocked the other monster down and punched him multiple times. Finally they split apart and Monster X and Godzilla locked beams, causing a massive explosion which blew the kaiju off their feet.

Godzilla staggered to his feet, and looked up to see Monster X undergo a terrifying transformation; he dropped to all fours, grew wings, enlarged his bodily proportions, changed his white armor into a dark golden armor, and then finally, his neck elongated and his shoulder skulls filled out and became a pair of extra heads. Monster X had transformed into Keizer Ghidorah (カイザーギドラ, 'Kaizaa Gidora').

Keizer Ghidorah
The final battle for the fate of the planet had only just begun. As the Xiliens' reign of terror was finally coming to the close, the showdown between Godzilla and Keizer Ghidorah had just begun. It appeared as though Godzilla was going to procure an immediate advantage, but the space demon turned out to be much stronger than anticipated. His three gravity beams tore into Godzilla and knocked the proud creature to the ground. Godzilla was dragged across the entire city by the saffron streaks of lightning, and Keizer Ghidorah proceeded to sink his teeth into the gargantuan guardian, absorbing the very life force out of Godzilla. It appeared as though Keizer Ghidorah would soon become the new monster king; it appeared as though Earth would soon suffer the full wrath of this terrible tyrant.

Humankind had something to say about it, however. The Gotengo charged and fired a beam at Godzilla. This was no ordinary beam, however. Godzilla was given the power of Keizer, an ancient energy procured from the Xiliens that would give extraordinary abilities to whomever possessed it. With this power, Godzilla was able to send a pulse of energy throughout his body, releasing Keizer Ghidorah's tight grip. The revived reptile fired his atomic ray at Keizer Ghidorah's middle head, causing it to explode! The cunning creature forced the hateful hydra's right head to send a gravity beam scorching into its left, leaving the grotesque dragon mangled and maimed. The limp body of the once tyrannical titan was hurled into the sky, and a hyper spiral ray followed. The motionless body flew far into the atmosphere and erupted into a million pieces. Godzilla roared in victory, for this awful oppressor was finally defeated.

Monster X

 * Like King Ghidorah, Monster X can fire Gravity beams, but from his eyes instead of his mouth(s).
 * Monster X has limited levitation.
 * Monster X has the ability to transform into Keizer Ghidorah.
 * Monster X is very agile and strong, giving him great martial art skill.

Keizer Ghidorah

 * Like King Ghidorah, Keizer Ghidorah is able to fire Gravity beams from his mouths.
 * Keizer Ghidorah can drain energy through his bite.

Trivia

 * It is possible that Monster X's name is a reference to Monster Zero, another alias for King Ghidorah.
 * Monster X is also used to refer Guilala.
 * Monster X was considered for Godzilla: Unleashed, but he was "not interesting enough", and his transformation into Keizer Ghidorah was too complex.