A rare substance in the core of a comet near the Bajoran wormhole, whose trail created a subspace filament that carried a signal to the Gamma Quadrant and fulfilled Trakor's prophecy in Destiny.
Silithium is a material found in the comet at the centre of the Deep Space Nine episode Destiny in 2371. Its unusual subspace properties make the comet a danger to the wormhole, since a large amount passing through could cause it to collapse.
A joint Federation and Cardassian project, with the scientists Ulani Belor and Gilora Rejal, is trying to set up a communications relay on the far side of the wormhole. Vedek Yarka warns Sisko that Trakor's Third Prophecy foretells disaster when three vipers return to their nest in the sky. When the Defiant tries to deflect the comet, a sabotaged phaser shatters it into three fragments.
Sisko and Kira guide the pieces through inside a shuttlepod's field, but some silithium leaks out. Instead of harming the wormhole, the trail forms a filament through it along which the relay signal can travel, fulfilling the prophecy benignly.