Orb of Souls

The Orb of Souls is one of the nine Tears of the Prophets, taken by the Cardassians during their occupation of Bajor, and eventually returned to Bajor in 2376 along with the Orbs of Destiny, Truth, and Unity, thus returning all nine Tears to Bajor. (DS9 - Mission Gamma novel: Cathedral)

History

According to Opaka Sulan, the Orb of Souls is the least understood of all nine orbs, and encounters with it are also more tasking than any of the other orbs. Not many willingly look into the Orb of Souls, though it has been known to 'call' to individuals as a sort of summons. (DS9 novel: Fearful Symmetry)

An unknown number of centuries prior to the Cardassian Occupation, a Bajoran Kai named Dava Nikende was called to the Orb of Souls, and encountered it. (DS9 novel: Fearful Symmetry)

First splinter timeline

In 2376, the Orb of Souls and the others were discovered in an Obsidian Order bunker in Cardassia City's Munda'ar sector. Elim Garak later brought Yevir Linjarin to the bunker for him to take possession of the Orb and the others. Yevir later unveiled the Orb of Souls on Deep Space 9. (DS9 - Mission Gamma novel: Cathedral)

In January of 2377, Benjamin Sisko consulted the Orb of Souls on Bajor. His encounter led him to a setting similar to where he had previously spoken with the Prophets, but where he now met with seven other Benjamin Siskos, each from a different alternate universe. The experience allowed Sisko to glimpse the entire life stories of his alternate selves, who explained to him how all Siskos in all alternate universes were all supposed to be the Emissaries to their respective alternate Bajors. (DS9 novel: Fearful Symmetry)

The same property of the Orb of Souls was responsible for allowing the respective Iliana Ghemors from "our" universe and the "mirror" universe to encounter one another, learn of each other's plans, and set in motion plans to stop each other. (DS9 novel: The Soul Key)

Mirror Bajor's Orb of Souls is the source of the object known in our universe as the pagh-varam, the "soul key." In order to protect them from the Terran Empire, the alternate Kai Dava defaced all the Orbs of his Bajor, leaving only fragments of each one intact. During his encounter with the Orb of Souls, the alternate Dava was somehow able to transfer the fragment of the Orb of Souls to this universe's Dava. Presumably the alternate Davas were able to explain why this was a necessary part of the Prophets' plans for Bajor, as the alternate Siskos had done. (DS9 novel: The Soul Key)