The Reckoning Tablet was an ancient Bajoran artifact discovered under the ruins of the city of B'hala in 2374. The tablet was inscribed with archaic Bajoran ideograms that described the upcoming Reckoning.
After the tablet was discovered it was taken to Deep Space 9 by Captain Benjamin Sisko for study. After this the Bajoran wormhole entered into flux, causing environmental instability on Bajor, causing Kai Winn Adami to travel to the space station to request the tablet be returned. Frustrated by his lack of understanding of the tablet Sisko broke it, freeing a Prophet and a Pah-wraith.
In an alternate universe where the Dominion won the Dominion War, the tablet was found by Jake Sisko and Prylar Letac Vari after Jake received a vision during an orb experience. Jake and the tablet were taken to Gateway Station where the tablet was confiscated by the Dominion. Weyoun wished to present the tablet as a fake with Jake Sisko as he believed that the tablet would foster rebellion against the Dominion.
While having dinner with Weyoun in an attempt to retrieve the orb Jake accidentally broke the tablet in a fit of rage and freed the two entities contained within and starting the Reckoning.