Age of Myth
Age of Sigmar
During the Dread Solstice, Nagash sent armies marching to Nagashizzar to stop him using gaping fissures that were gates to the Great Outbliette. However he was too arrogant, distracted and set off balance by the amounts of armies assailing him. Eventually the silent halls of the prison becomes a battlefield and hundreds of cell are broken open by weapons the prison's arcane warding cannot resist. With the souls released they begin to recorporeate, releasing prisoners, like ancient magisters and mages, disgraced vampiric aristocrats, snarling greenskin warlords, ice-hearted queens and Sigmar-touched heroes, blind loremasters, duardin kings and arrogant Skaven clan-lords some from even before the Age of Myth. 
The anarchy of battle escalated by these resurrections and while some were once again imprisoned many other managed to escape. This prison break saw the return of many of the greatest heroes of Chaos, Order and Destruction as well as huge reinforcement to the Anvils of the Heldenhammer as Sigmar quickly snatched many heroes with his lighting.