The Council of Twelve was a government body within the government of the Star Imperium.  It was selected from the ranks of the Imperial Parliament by random lottery and Council members served one-year terms of office. 

The Council of Twelve served as supreme court of the Star Imperium.  It also assumed the powers of the Imperial Sovereign when that office became vacant or when the ruling Sovereign was declared unable to exercise his or her powers and duties. 

The Council of Twelve continued under the United Planets, and it's members served one-year terms, selected by random lottery from the ranks of the United Planets Assembly.  The Council continued as the intergalactic supreme court while it had the authority to veto bills passed by the Assembly.