Nibiru

Nibiru is a legend well-known in Babylonian history. 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, god-like creatures on the planet Nibiru will arrive on Earth. One known sentient of the planet was captured inside a sarcophagus. A planispheric disk was created in order to make the treasure hard to find, first created by the Conquistadors who realised the power of the god inside.

Mentions
The first mention of "nibiru" was unseen by Mystery Incorporated, but seen to the viewer as the ghost of El Aguirre, leader of the Conquistadors as they left Haunted Mountain. (Night on Haunted Mountain)

First mentioned in front of Mystery Incorporated was Frau Abigail Gluck, her last dying word. (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 underground uttered the word "nibiru". (Wrath of the Krampus)

After solving a mystery, the real Mummy Friar Serra returned to warn them of "nibiru", a cataclysm that is inevitable unless "the dog dies", to stop nibiru, the dog must die. (Theater of Doom)