Pillar of Commandments
The Pillar of Commandments is a pillar of the purest Warpstone. It was placed in Skavenblight by the Horned Rat himself  in 2302 IC. It has 13 sides and each of the sides contains 13 commandments, creating a total of 169 edicts .
Their exact text is unknown, but the edicts seem to dictate the rules of Skaven society, and especially of the ruling Council of Thirteen.
Any Skaven with ambitions to become a member of the council must touch the pillar. Most will perish immediately, but some have survived. Any survivor must then challenge an established member of the council to a fight to the death. If he manages to defeat and to kill his opponent he will take over his place.
No Skaven has defeated a member of the council  for the last 200 years.