Pillar of Commandments

Each position within the Council of Thirteen is open to new potential candidates or replacements at any given time. In order to become such a candidate, the applicant must place his hands upon the Pillars of Commandents, which would signify the candidate as being worthy to become one of the Lords of Decay. Then, the candidate must simply challenge one of the already existing members of the Council in a fight to the death. If the candidate wins, he takes the place of the loser upon the Council Chamber. Though rare, more than one member of the Council has been replaced this way.

Source

 * Warhammer Fantasy RPG 2nd ED -- Children of the Horned Rat
 * pg. 44