Where did I say that anyone was siding with Nagash? you seem to be confused. All I said was that the some of the Katophranes may have been imprisoned in the Great Oubliette.
This is from the end of the campaign.
THE GREAT JAILBREAK
As battle rages through the Prime Innerlands, Nagash casts ever more enemies into the Great Oubliette. Warlords and warriors alike tumble helplessly down hissing wells of grey sand, before plunging into the deeper darkness below. They stagger to their feet amidst endless stone corridors and whispering walls of shadow.
In his arrogance, distracted and set off balance by the sheer amount of foes hurling themselves at him from every side, Nagash has erred. More and more armed warriors mass in the corridors and halls of the oubliette until at last its guardians cannot hold them back. War rages through halls that have known naught but silence for thousands of years. Weapons are unleashed whose power even the arcane wardings of the oubliette cannot withstand.
Hundreds of cells are smashed asunder and the souls within released to recorporeate. Occupants trapped since before the Age of Myth emerge, blinking in the light. 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; all of these and countless others burst from captivity and seek to make good their escape.
Anarchy reigns as battles are interrupted by the resurrection of beloved heroes, the return of long-lost loved ones and the sudden recorporeation of nemeses long thought slain. Some souls are trammelled once again, of course, but many more escape.
So are the ranks of the Anvils of the Heldenhammer hugely reinforced, Sigmar’s lightning spearing down again and again to snatch up heroic women and men loosed from millennia of hardship. So too do many races see the return of heroes enshrined since time immemorial in folklore and legend. None can say what fate the Mortal Realms will face in the dark days to come, and all attempts to unseat Nagash have failed, but at least the forces of Order, Destruction and Chaos alike will face the looming darkness with their greatest heroes restored to them.
You were an instrumental part of this great victory, warlord, and you stand taller in that knowledge. Let the shadows gather – you are ready.