A floating black cube from another plane, and one of the six artifacts for unlocking Brizoor Waal.

When the verse is complete, the writing desk opens and an onyx cube appears before you, levitating in the air. The library shakes and crackling black energy fills the air above your head. - Outbreak in Undercity (RPA-ADV3)

The Onyx Cube, stored away in the durgolium desk of Bertram Hoogna in the Undercity Library, is revealed to be one of six keys for opening Brizoor Waal.

It is obtained by returning an inkwell of blood to Bertram's desk, upon which a message is written by his quill;

Thank thee kindly for the blood
I shall not ask about its source
As my gratitude does bud
I hereby yield a wondrous force
This cube is ancient as can be
Keep it safely in thy care
It levitates most marvelously
And opens doors that lead elsewhere

The cube now secured in your pack, you feel strangely lighter on your feet, and lighter in your spirit as well. - Outbreak in Undercity (RPA-ADV3)

When traveling to Moti, Plane of Death, the Cube is used to complete the obelisk there;

…and releases the onyx cube, which shines a pale blue beam of light toward the obelisk, revealing a small, square gap in one of its sides. The obelisk immediately stops rotating and remains firmly facing the cube. The horrible stillness of this plane is now complete.

With the onyx cube in place, the obelisk is complete. It opens slowly, with each of its faces unfolding like the petals of a flower. A painfully harsh light and wind race forth from the first cracks between these petals, and their force shakes the entire plane with violence. The sky is filled with a strange snow of spirits, awake now and softly drifting downward. When they touch the ground, the world is flooded with color and visions from their many lives. - Gate of Brizoor Waal (RPA-ADVF)