Dark Jedi Primarch
From Emperor's Hammer Encyclopaedia Imperia
Dark Jedi Primarch is a rank in the Dark Brotherhood reserved for only the most powerful Dark Jedi to ever be a Dark Jedi in the Emperor's Hammer's Brotherhood.
It's the maximum rank anyone can ever even hope to achieve without being the Grand Master, and the maximum rank for the Deputy Grand Master position.
Only one rank trumps the Dark Jedi Primarch, and that rank is Grand Master.
