The Nibiru cataclysm is a supposed disastrous encounter between the Earth and a large planetary object (either a collision or a near-miss) which certain groups believe will take place in the early 21st century. Believers in this doomsday event usually refer to this object as Planet X or Nibiru. The idea that a planet-sized object will collide with or closely pass by Earth in the near future is not supported by any scientific evidence and has been rejected by astronomers and planetary scientists as pseudoscience.
It is said that the Nibiru planet is roughly four times the size of the Earth, and while its exact approach towards the planet unknown, many said that its closest approach would occur "very soon", resulting in the Earth's rotation ceasing for exactly 5.9 terrestrial days. This would be followed by the Earth's pole destabilizing in a pole shift, a physical pole shift, with the Earth's pole physically moving, rather than a geomagnetic reversal, caused by magnetic attraction between the Earth's core and the magnetism of the passing planet. This in turn would disrupt the Earth's magnetic core and lead to subsequent displacement of the Earth's crust.
Even if the planet would miss the Earth, this does not spare it from the asteroids that encircle it. For it is said that as Nibiru courses through the Solar System on its way to Earth, it will attract asteroids or perhaps even small moons from other native planets due to its powerful magnetic fields and gravity. As a result, for even if the planet misses its trajectory, the asteroids will finish the job. As analyzed by astronomers, the asteroids that Nibiru attracts would be have an explosive force roughly equal to that of the strongest hydrogen bomb. For when they strike the planet, they will cause nuclear winter, colossal tidal waves and floods, and an extreme amount of property damage.
The Nibiru cataclysm is said to be caused by the Anunnaki, a race of extraterrestrial god-like beings who visited Earth around 25,000 years ago, and offered extensive knowledge and detail to the developing humanoids; whom at that time did not have the brain capacity or thinking power to comprehend what the Anunnaki were even saying. This event is likely due to the fact that the Anunnaki were observing the humans use of the knowledge that they provided them with, and when they witnessed them using it for selfish and destructive means, the Anunnaki were furious. As a result, these alien deities decided to plunge their very planet into Earth itself. Why they would do such a thing to their own home world is uncertain, but according to one of the Anunnaki, it says that its world "is nothing compared to the many that they hold". Highly implying that they inhabit many more planets not native to the our solar system.