The Grey Lords were the very first chosen of the Horned Rat, a result of the lack of guidance the first few generations of Skavens endured during their ill attempt at breaking open fissures within the earth during the Time of Woe incident. There are very scant few records detailing the 13 Grey Lords that emerged through the Horned Rats chapel, for nearly two millenia had passed when the event occurred, and the majority of Skaven lack the motives or desire for written history. All that was known was that the Grey Lords were Horned skavens, similiar in appearance to the current day Grey Seers. They had also acted as the forerunners to the first Council of Thirteen, a position of power each and every Grey Lord was allowed to partake. To this day, there are no currently living Grey Lords, for all have either been dead or killed off during the past few centuries, for even with the immense power the Grey Lords had within their disposal, they are still only mortal.
Source
- Warhammer Armies: Skaven (7th Edition) pg 19 - 20