The Uprising was a revolt by the Circle, led by Valentine Morgenstern, against the Clave and Downworlders during the signing of the Accords in Alicante in 1998. The confrontation led to a fight that resulted in bloodshed, though its effects were thwarted and limited because of the efforts of Valentine's wife, Jocelyn, and former parabatai, Lucian.
Some time before the signing of the Accords, Valentine had made it clear that he was against the signing of the peace treaty with Downworlders. Valentine and his followers were also advocates of using the Mortal Cup to create a new Shadowhunter army. When the Clave did not heed his words, Valentine and the Circle began plotting an Uprising against them. Knowing the Cup would be on display at the signing of the the Accords Hall, they took the opportunity to partly accomplish three of their goals: destroy the Accords, murder Downworlders, and get ahold of the Cup.
On the day of the signing, Valentine and his followers infiltrated the Hall and went on to slaughter the people inside who went against him. The Uprising ended in bloodshed and massacre, and Valentine was able to get the Cup, but once outside, he was intercepted by Luke, and later Jocelyn, who took the Cup and left.
Valentine staged his death and went into hiding after the Uprising. The members of the Circle who did not leave before the Uprising faced trial and punishment.
For years after that, the Uprising became a horrible part of Nephilim history. Because of this, while the general idea of the Uprising and Valentine's followers is known by most Shadowhunters, the Circle and the specifics of the event are mostly unknown, specifically among the younger generations, because the Clave had tried to rid their history of this embarrassment and made it mostly forbidden to talk and even hear about the Circle.