Either a menacing infra/red, invisible celestial heatsource near our solar system, or some divinely ordained celestial object of wrath in an unseen realm, able to manifest physically if karmic forces within humanity warrant this cleansing, vengeful celestial manifestation.
Maybe this Niburu scenario was averted for humanity in 2003 by divine grace, when this invisible celestial energybody was apparently present in our solar system beyond Jupiter according to different sources.
I believe some of the mystical catholic writings mention the possible emergence of such a celestial object of wrath from the Unseen in case of an unrepentant mankind.
Anyway, that´s my likely fanciful take on the Niburu subject!