From Akashic Records2012. The whole theory started with the predictions of Nancy Lieder. It is widely regarded as bullshit, but because of the numerous anecdotal evidence it is allegedly has, it is still discussed quite often on /x/.
The common claim
The most prominent claim mirrored on the threads is that Nibiru is a brown dwarf with a highly elliptical orbit that periodically visits the inner solar system. It is said that it will pass by Earth somewhere between the 21st to the 25th of December 2012. While some claim that it is already observable, no believable evidence of this has been presented to this date. Others claim that Nibiru is too far away to be observable yet, but will be in the near future.
It all started when a woman named Nancy Lieder founded the website ZetaTalk. She describes herself as a contactee who has the ability to communicate with aliens called Zetas through an implant in her brain. The so-called aliens then told her that she had been chosen to warn mankind about an upcoming catastrophic event - namely that a planetary object would sweep through our solar system and wipe out most of humanity. The time she claimed that this would happen was the year 2003. Clearly nothing of this kind happened, so she revised it to 2010. The year 2012, however had at first no connection to this theory until it was merged with the 2012 doomsday prediction.
Varying theories and evolution
The whole Nibiru collision hypothesis has a quite complex background of various events and theories, and thus different people understand a different concept behind the event.
Merge with the 2012 doomsday prediction
Merge with various Bible cults
Because both the Nibiru and Long Count calendar talk about an apocalypse, the whole thing was of course attractive to the religious. There is a certain Bible cult or movement that combines the teachings of the Bible and the conspiracy theory of so-called Reptilians into one conspiracy theory. This theory was combined with the Nibiru doomsday prediction as they both predict that the end of the world is near.
Lack of evidence
The main argument against the theory of an object with a highly elliptical orbit is the lack of evidence. Only anecdotal evidence is presented, like pictures of various brighter nightsky objects like Jupiter or Venus or daylight pictures of lensflares from the Sun. If you see a bright object at night, first check whether it's not a planet (Jupiter and Venus are very bright) or one of the brighter stars (Sirius, Arcturus, Vega, etc). You can do so by using nightsky software, both offline and online.