Planet X (Cybertron)
There is no longer such place as Planet X. It continues to exist solely as a codeword, an identifier for an underground army of loosely-affiliated Transformer criminals. The true number of "Planet X's" forces is unknown, and many of their identities are similarly shrouded in mystery.
What is known about Planet X is that the warriors, thieves, assassins, and other dubious characters who claim a link to it are the survivors of a dead planet. The Manifest Destiny of Planet X was to subjugate countless worlds, and it did, enslaving the natives and feeding the whole of their conquered worlds piece-by-piece into the World Furnaces that powered it. But their expansion ended at contact with Gigantion, the Giant Planet. As a result of the battle between the two planets, Planet X went nova. The few survivors, driven mad in their quest for vengeance, made a deal with no less a force than Unicron himself, offering their services in exchange for the destruction of the hated Giant Planet.
According to the booklet included with the final Galaxy Force DVD release, Planet X itself was the living body of a Unicron from a parellel universe that went into a frenzy repair mode after Master Megatron absorbed Unicron's spark. Planet X's repair systems went mad and resurrected a portion of Transformers it had absorbed from Cybertron to manage it's frantic systems. These beings were the agents of Planet X and unknowing servants of Unicron. For decades, they took over other worlds for fuel in a galactic conquest.
Soundwave, a data analyst dug deep into their planet and found a massive memory bank belonging to Unicron. Soundwave and the others moved the data to their own memory banks and began to study it. Planet X eventually travelled to a parallel universe in search of energy and encountered Gigalonia, eventually unlocking the full potential of their planet, and destroying themselves in the process. Long after Planet X's explosion, Soundwave was able to learn of Primus, the keys, and their ties to Gigalonia. Soundwave and the few survivors of Planet X are looking to use these things to resurrect their homeworld and destroy Gigalonia. Shortly after the conclusion of Galaxy Force, Soundwave and Sideways escaped fire space to examine the dimension they had been transported to after the climactic explosion between Master Galvatron and Super Starscream. In this universe they found another Planet X, not yet destroyed.
The Decepticons Sideways and Soundwave are currently the only known Planet X operatives.
Note: If Soundblaster is indeed a new character, he is certainly a Planet X agent as well. It is also theoretically possible that earlier Unicron Trilogy characters with Unicronian affiliations, such as Nemesis Prime could be connected to Planet X.
