The Dark Council are the group command staff of the Dark Brotherhood.
It has, over time, consisted of many different positions, most of which have been merged into others or disbanded:
The Grand Master is the group leader of the Dark Brotherhood. He is the highest authority in the Brotherhood. He is appointed by the Fleet Commander and Executive Officer. Aside from the title of 'Grand Master', he also holds the rank of the same name, and the title Dark Lord of the Sith, regardless of his original order alignment. He is also Governor Plenipotentiary of Eos, and usually has an Admiralty rank in the Emperor's Hammer. Some of the past Grand Masters have also used a Darth name, a right which has been ignored in recent times.
Deputy Grand Master
The Deputy Grand Master is the second-in-command of the Brotherhood. He is the chief apprentice of the Grand Master, and is usually, but not always, the successor to the mantle of Grand Master when the time is right. Along with the Grand Master, he was the only member of the Brotherhood allowed to appoint Consuls, but the actual duties of the Deputy Grand Master have varied under different Grand Masters.
Master At Arms
The Master At Arms was basically in charge of keeping the roll of the Brotherhood updated. It was the Master At Arms that would approve the majority of promotions, and, later, medal awards within the Brotherhood, and had full authority with denial of these requests if deemed unacceptable.
The Seneschal was tasked with keeping the Brotherhood's website running effectively, as well as improving it with new coding.
Each order within the Dark Brotherhood had its own leader. These were the Sith High Warrior, Obelisk High Commander and Krath High Priest. As well as being considered a third summit member for their order-aligned houses, they would release order-wide competitions based on the formerly used order platforms.