Tenth Planet

Aside from giving birth to an evil death phoenix, the Tenth Planet was pretty boring.

The Tenth Planet (第十惑星 Daijū Wakusei) is an undiscovered planet "in the far reaches of the Milky Way". It is the birthplace of Violengiguar.

Fiction

Zone story pages

The Zodiac reformatted the Tenth Planet into San Francisco, then?

For unknown reasons, the spirits of many slain Decepticon warriors fused together into the entity known as Violengiguar on the Tenth Planet. He proceeded to cover the world in dark clouds, blotting out all starlight and turning it into a barren, desolate world. From then on, he would use it as his base of operations. Zone Part 10 Zone Part 12

After seizing the Zodiac from the Autobots, Violengiguar returned to his birthplace. The Powered Masters pursued him and managed to defeat his three insectoid forms, allowing them to reclaim the Zodiac, Zone story page #11 but Violengiguar was not yet defeated, and revealed his true form as a flaming, phoenix-like mass of Decepticon spirits. The Powered Masters placed their hands upon the Zodiac and channeled its energy into themselves, then united into their combined form of Big Powered to destroy Violengiguar with the Zodiac-powered "Rainbow Powered Attack". With Violengiguar eliminated, the rainbow of Zodiac energy transformed the Tenth Planet into a paradise for human and Transformer alike. Zone story page #12

Legends comic

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Ask Vector Prime

Vector Prime stated that the Tenth Planet was in fact the body of Unicron from Primax 886.0 Gamma. After being blasted out of that dimension by Rodimus Prime with the Matrix of Leadership, Unicron's body ended up as the tenth planet in the Sol system in Tyran 609.08 Kappa. The binary end-state nature of that universe put too much strain on Unicron's systems, and his comatose form resurrected many of the Transformers who he had consumed. They dubbed their world Planet X, and journeyed to Primax 290.0 Zeta to begin a campaign of conquest. However, the local Autobots thwarted the denizens of Planet X and used the power of the Zodiac to eliminate Unicron's essence, splitting the planet in two. Half remained behind as a paradise planet while the other was shunted to Aurex 105.08 Alpha, where it finally met its end in war with Gigalonia. Ask Vector Prime