Nibiru Entity

The Evil Entity is the true antagonist in ''Scooby Doo Mystery Inc. ''He is an evil Annunaki that wants to destroy the world. He was imprisoned in the treasure of Crystal Cove but was released by Pericles.

info
Like other more benevolent members of its race, the Evil Entity came to Earth thousands of years ago at the time of Nibiru, when a planetary alignment weakened the barriers between dimensions and allowed them to cross over. While the other Anunnaki sought to help humankind grow, the entity desired only destruction. Its plans discovered before it could take physical form in the Earth dimension in the manner typical of its race—by possessing a native animal—the entity was trapped within a crystal sarcophagus filled with black pearls by the other Anunnaki. Through means unknown, the sarcophagus wound up in the Yucatán, where the entity devised a plan to ensure its release. Using what little psychic power it was still able to affect on Earth, it subliminally brought together the Hunters of Secrets—a group of four humans and their pet jaguar—with the intent that they locate its sarcophagus and free it. The Hunters deduced the entity's true nature and plans, however, and devised a means to destroy it using the Heart of the Jaguar, but as they were making preparations, the sarcophagus was stolen away by a band of Spanish conquistadors led by Fernando El Aguirre, who saw it as a valuable treasure—perhaps as the result of further manipulation by the entity in order to save its own life.

The entity's mere presence proved corruptive to the conquistadors, who succumbed to dark impulses, driven to destroy entire villages in search of riches. Deeming the sarcophagus cursed, El Aguirre and his men anchored in a cove on the North American coastline and took the sarcophagus and all the other treasure its influence had compelled them to acquire deep into catacombs beneath the bay, intending to hide them away forever. Using the mysterious, alchemical Planispheric Disk, they charted a path through caverns, and through time and space itself, sealing the sarcophagus and the entity away in the space between dimensions, barring the way with four elementally-locked doorways, and then splitting the disk into six pieces and scattering them to prevent the "cursed treasure" from ever being found again.

Although sealed away, the entity's corruptive influence remained strong, and its mere presence shaped the town that grew on the land above its resting place, dubbed "Crystal Cove" by the conquistadors for the dark secret it held. Under the entity's sway, the town became a centre of the mysterious and weird, where people decided that the best way to commit crimes was to dress up as ghosts and monsters using increasingly outlandish and infeasible technology. But these mystery scenarios were but the catalyst for the entity's true plan: as it had done before with the Hunters of Secrets, it brought together groups consisting of four humans, whose mystery-solving abilities would allow them to discover the locations of the scattered pieces of the Planispheric Disk and uncover its prison, and one animal, which the entity could then possess in order to take physical form on Earth. Unfortunately for the entity, its corrupting touch also worked against it, as group after group fell victim to their dark sides, turning on each other and falling apart long before they could find all the disk pieces. The group that came the closest was the original Mystery Incorporated, whose maniacal mascot Professor Pericles actively sought to be possessed by the entity in order to gain its power, but they were forced out of Crystal Cove by Fred Jones, Sr..

When the new Mystery Incorporated succeeded in locating all the disk pieces and reforming the Planispheric Disk, they were forced to hand the disc over to Pericles, who used it to unearth the elemental gates hidden beneath Crystal Cove. Pericles liberated the entity, who intended to possess Scooby-Doo, but Pericles offered himself up to the entity as a willing host, evidently unaware that he would be entirely subsumed by the entity upon possession. Taking Pericles's body, the entity gained physical form as a tentacled mass and set about consuming all of Crystal Cove's inhabitants before moving on to consume planets and galaxies. Discovering that the entity was unable to hurt them directly, the Mystery Incorporated kids realised that unlike the other mystery-solving groups forged by the entity, their friendship was real, like the Hunters of Secrets before them. With this revelation, they plunged the Heart of the Jaguar into the sarcophagus, severing the entity's link to their dimension and destroying it. The obliteration of the entity retroactively wiped all trace of it from history, creating an alternate timeline in which all those who had fallen victim to its "curse" lived happy and contented lives.

Role in Ultima
Although the Nibiru Entity was destroyed for good and its influence removed from Crystal Cove for good, a small piece of the dark god still exists somewhere out in the universe....

Nibiru is even more important to the Scooby Doo mythos than just the Mystery Incorporated series. In fact, he's responsible for everything wrong in ALL of the Scooby Doo universes. For instance, he disguised himself as a Cat God to con and control Simone Lenoir, Lena Dupree, and Jacques the boatman, imprinting his face onto their werecat forms as a symbol of his control over them. He gave Sarah Ravencroft the knowledge necessary to create her Dark Spellbook, just before imprinting his face on her as well to control her ghost, and then manipulating Ben Ravencroft into searching for her. He even went inside the dreams of the SALF team, Bill, Amazing Krudsky, etc. and influenced them all into creating their monster costumes and plans through inspiration of the many demonic servants he wielded under his command. In fact, the Goblin King from "Scooby Doo and the Goblin King" was among the many gods that were responsible for trapping him in the coffin in the first place!