Why not fluff it like this?
Once, in a city both ancient and learned in the magical arts, there was a plague so virulent and unstoppable that within weeks, 90% of the city had perished. Though the nature of the plague has been lost in time, it was recorded that it was beyond natural fevers, unmoving in the face of magic or faith. Though the Clerics and Wizards of the city employed both heroic and harsh measures to stop the disease, it flowed through quarantines and outran every attempt to halt it, killing thousands in the first day alone. How such a plague was unleashed, and why it never flourished outside the city, are both facts unknown. However, it is known that there were survivors, 206 of them, members of the secretive 'College of Bone'. The number held special significance in their branch of magic, which can only be guessed upon. Although it has been rumored that the survivors and perpetrators were one, the College was close-mouthed as ever, and few dared the gates of the dead city.
In the ashes of their ancient metropolis, they raised an army of the dead, ostensibly to guard the treasures that the city had once held, and in this city of dry bone and derelict acropolis, the 206 liches of the College of Bone lived in harmony, working together on a great and secret project. Through the millennia, their forms wore away, and they ascended to demi-lichhood. However, this was not their true aim. The nature of the experiment is something lost in the ages, for those that knew felt it too unnatural and obscene to record.