Nibiru

Nibiru is a legend well-known in Babylonian history.

Legend of Nibiru
It tells the story of how a large object, either a planet known as Twelfth Planet, Planet X or Nibiru, will collide with Earth. It is referenced as "The Doomsday Event". In this event, deity-like creatures called the Anunnaki from the planet Nibiru will arrive on Earth.

Actual history
One known sentient of the planet was somehow captured inside a sarcophagus; it is unknown how and why it was imprisoned. This Anunnaki, the most evil of its kind, has for aeons plotted its revenge, possessing animals in attempts to free itself.

Spanish Conquistadors who were told of a crystal sarcophagus filled with black pearls found it, but the being inside possessed and controlled them, driving them insane and corrupting them as they ransacked the villages of former allies in search of treasure. However, they were eventually able to resist the Entity's influence enough to hide the treasure under what became later known as Crystal Cove; realizing the power of the Entity, they used a complex guide to mark the location of the sarcophagus, then broke the guide into six pieces, buried in six locations, to prevent people finding the Cursed Treasure. The Conquistadors then scattered to the four winds, as were the pieces of the Planispheric Disk.

The first Mystery Incorporated eventually managed to find one of the disk's pieces before being disbanded; the current group have been working to gather and decipher the disc's pieces, discovering clues to keys from ancient Maya times along the way. More recently the word "Nibiru" has appeared in almost all of the incidents, being uttered by the deceased or even decoded from the Planispheric Disk.

Season two
"Nibiru" spoke through the ghost of El Aguirre, leader of the Conquistadors as Mystery Incorporated left Mount Diabla. (Night on Haunted Mountain)

Abigail Gluck said it when Mystery Incorporated found her corpse. (The Midnight Zone)

Velma Dinkley searched on her website for any information on Nibiru and discovered it to be the doomsday event. Unknown by Mystery Incorporated, after they connected the Planispheric Disk together, the sarcophagus in the lost underground city uttered the word "Nibiru" and laughed. (Wrath of the Krampus)

Velma cobbled together a player for the Planispheric Disk, and it produced the word "Nibiru" also.

After solving a mystery, the real mummy Friar Serra returned to warn them of "Nibiru", a cataclysm that is inevitable unless "the dog dies". (Theater of Doom)

Nova was put in the hospital, placed in a deep coma. During one of his visits, the monitor turned off showing that she died. Suddenly she awoke and warned Scooby-Doo, "Nibiru, Nibiru is coming". (Dance of the Undead)

An Anunnaki possessed Nova and appeared in Scooby-Doo's nightmare telling him that Nibiru is coming. She told Scooby-Doo to destroy the cursed treasure before the evil Anunnaki was released within the sarcophagus. (Stand and Deliver)

When the gang attempted to destroy the Evil Entity, Nibiru arrived and all the planets were aligned. (Through the Curtain)

Appearances

 * Scooby-Doo! Mystery Incorporated
 * 213. Wrath of the Krampus (video)
 * 224. Gates of Gloom (video)
 * 225. Through the Curtain
 * 226. Come Undone