Council of State was a governing body aimed to replace King Charles I.
John Thurloe considered it necessary to replace the king with a collective body to avoid the accusations of the Levellers that one king is simply being replaced with another. Others in Oliver Cromwell's close circle, including Henry Ireton, Thomas Culpeper and Cromwell's own daughter thought that a figurehead was needed to successfully rule the land. In December 1648, preparing the trial of the king, Cromwell was undecided between these two options and sought advice from the Second Doctor and Jamie McCrimmon, who were posing as seers able to tell the future. (PROSE: The Roundheads)
Behind the scenes
In real universe, the Council of State was established after the execution of Charles I, but was soon replaced by Cromwell as the Lord Protector.