A Numinos is an object that can be placed on the center of a Beacon, to focus its energy. The search for a Numinos is a central part of The Cycle, which means much of the information about such object is shrouded in mystery, and it is possible that a "Numinos" is a nonexistent concept simply fabricated by the High Ones to keep up The Pattern.
Most of what is known about Numini indicates that lighting a Beacon with one at its core banishes the High Ones, essentially breaking The Cycle. This is possible by focusing the energy of the Black Stones on the High Ones themselves, and as such, a Numinos must be related to the High Ones in some form. However, since all information about Numini comes from the High Ones themselves, it is possible that nothing can actually focus the energy of the Black Stones, and a Beacon will always trigger The Cleansing when lit, with or without a Numinos. Since no known civilization has managed to light a Beacon with a Numinos at its core, it's impossible to know for sure.
The only known Numinos is the Word of the Dead filled with the essence of a High One. This essence would essentially "guide" the energy of the Black Stones towards the High Ones, eliminating them. Acquiring such essence is possible by using the Word of the Dead itself to enter the mind of a High One, then activating it inside the High One's mind, to trap its "soul" inside the artifact. However, since no known civilization has succeeded in doing that, it's impossible to know if this "trapping" would work, or if it is just a made-up concept created to drive The Pattern.