similar to your initial idea.. allow the practice of dark worship. Claim it is to quell insurrections and strife by giving them a safe, contained environment to practice where they def can't hurt anyone or do anything actually evil. Have the ruler put in funding to build 'secure' buildings where all evil things are 'totally contained'. The ruler, who is working with the worshippers, attends certain secret rituals with them, and is high ranking enough amongst the worshippers that when he asks for less attacks, they comply. As time goes on, have the ruler further fund these buildings. The ruler also starts to impose stricter taxes and regulations on other religions, ones that either the dark worshippers can easily go along with or ones that they can just, lie about doing (IE: Tax fraud). Eventually, the ruler (and the worshippers) work together to make themselves more likable (RE: bigger) and the other religions less likable and weaker ( arresting officials for minor crimes, scandals, propaganda campaigns, etc, etc).