The Unnamed One is a character first mentioned in the Masters of the Universe minicomics who would later appear in other related media.
Minicomics
The Unnamed One was first mentioned by King Hiss in The Powers of Grayskull: The Legend Begins! When Skeletor time-travels from present day Eternia to Preternia and offers an alliance to the Snake Men of the time, King Hiss wonders if this stranger is an emissary to the Unnamed One who they serve.

Masters of the Universe Classics
An Unnamed One figure was released for Masters of the Universe Classics as an ancient renegade Trollan. His bio says as follows:

Formerly an Overlord of the Timeless Dimension, this great wizard betrayed his brethren when he attempted to steal the Power of the Universe for himself. The Aura of the Gods was infused into the Sword of He for protection and Gorpo was stripped of his form and banished to the Nameless Dimension. Here he became known as the Unnamed One, spoken of only in whispers and myth. During his exile, he discovered a method of combining DNA of several reptilian species. He used this knowledge to create an army of cold-blooded warriors which he commanded to raid magical planets in an attempt to consolidate power for his revenge. His evil schemes were at last undone after Dare, the new He-Ro, confronted him in single combat slaying the ancient mage with the very power he once attempted to steal.
Trivia
Emiliano Santalucia, when asked if the being inside the crystal of the first 200x Masters of the Universe comic series was the Unnamed One, mentioned that the Unnamed One's essence was split in two, one trapped in the crystal and the other taking a mechanical body and becoming Horde Prime from the She-Ra cartoon and UK comics.

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