The Deep Sovereign is an engimatic cult figure, rumoured to be the leader in a network of individuals who secretly favour spirit worship returning to Jovinium. Some members of the cult favour the use of pacting returning into common use, but do not harbour any sympathies for the Dominion or for Malvern.
Of course, the Deep Sovereign has been sought after by the Inquisition for generations. Sometimes, a person who might possibly fit the description is caught, tortured and executed - but there always seems to be a Deep Sovereign. Thus, it is likely it is a rank rather than a single individual.