This was an Orb of the Prophets. The Orb was sent from the Celestial Temple to guide Bajor. This was the third of nine Orbs that appeared within ten thousand years. One influenced Trekor, who became a poet, when he encountered one 3000 years ago.
It was kept under watch by Vedek Bevil Antos. In 2370 he consulted the Orb during the next Kai election. In 2373, Sisko got a plasma burst that gave him a vision in which he had an encounter with the Orb of Prophecy. In 2374 the Orb was moved to the Space Station. Bareil was going to steal it and travel to his alternate universe, but he left it after having an experiance with it.