have everyone whisper a personal code of their choosing to the king. Every day, they shall repeat the code. Make sure everyone remembers not to put it in their wills. Every day, people shall whisper the king the same code. Anyone who whispers the wrong one or doesn’t whisper shall be executed.
Currently, a bug surrounds the possessor possessing king so you should know when it happens. When it is fixed, have the king whisper his code to two different people.