From the Republic's vantage point, there's never any way to know for certain which Dark Lords of the Sith are seated on the Dark Council. The enigmatic and turbulent collective's roster is prone to constant change, making the task of keeping track all the more difficult. In fact, it was only very recently confirmed that a successor to the briefly tenured Darth Arho had been chosen.
Darth Arkous, from what little SIS has been able to gather, is more of a long-term strategist than his predecessor, opting for craftiness over belligerence. Because of this, others within the Empire have reportedly underestimated Arkous as a threat‒and have ultimately suffered for it. With Special Forces heavyweight Colonel Darok on his side, Darth Arkous may now be the most dangerous Sith in the galaxy.