Each one of them was a non-Awakened undead with an almost full red blood core and centuries of experience.
The Courts couldn't risk someone having cold feet at the last minute and defecting so no one had been forced in any way to take part in the mission and the members of the assassin squad were free to walk away until the mission started.
Each one of the undead had accumulated so much life force that their mass was on par with an Emperor Beast and even though they could only use true magic for the elements their undead species was naturally attuned with, they had survived the encounter with bright violet cored mages.
The fake mages were all dead and buried whereas killing an Awakened was harder, but the convened undead had all put down more than one cocky Council member each during their undeath.