The Order of Sith Lords stands atop the Tripartite Path as the ruling order. Membership in the Order of Sith Lords is reserved for the Dark Lord of the Sith, his predecessors, and those select few who have earned the right to bear the title of Sith Lord. Shadow Hands are considered junior apprentices of the Order.
As the Master of the Order of Sith Lords, the Grand Master serves as the ruler of all three Lesser Orders; the disciples of the Sith Order, and the two Sith cults, the Krath Order and the Obelisk Order.
While the Lesser Orders each carry only fragments of knowledge of the Old Sith Empire, the Order of Sith Lords represents the reunification of the Sith cults, mastery of Sith knowledge, and ascendancy over the Tripartite Path. The Sith Lords are the true masters of the Force.
History
Post-Palpatine Sith
Lumiya
Even with Palpatine's final death, Sith continued to survive. Their next Sith Lord was an Emperor's Hand named Lumiya. Darth Vader had passed on to Lumiya some Sith teaching secretly from his master, and the woman adopted the title Dark Lady of the Sith upon her masters' death at Endor in 4 ABY. She trained two apprentices: Flint, who was redeemed by Luke Skywalker, and Carnor Jax, who was killed by Palpatine loyalist Kir Kanos shortly after the Emperor's final known death on Onderon. Afterwards, Lady Lumiya went into hiding and it was thought once more that the Sith had disappeared.
In 40 ABY Lumiya had returned to the galactic stage, playing a role hidden from most of the participants of the Second Corellian Insurrection. Among her activities were the attack against the peace delegations that had gathered on Toryaz Station and the strange activies that followed on Lorrd. Soon afterwards she succeeded in convincing Jedi Knight Jacen Solo to join the ranks of the Sith.
Kane
Lumiya was not the only Sith to survive the Palpatine's death. Kane was another such follower. Like Lumiya, Kane had been one of the Emperor's Dark Side Adepts, and, like her, he too had recieved some Sith training. Unaware of Lumiya, following Palpatine's final demise at Onderon in 11 ABY and the destruction of Byss, along with most of the remaining Dark Side Adepts, Kane declared himself Dark Lord of the Sith and founded the Dark Brotherhood.
Sith Lords in the Dark Brotherhood
In the Dark Brotherhood the Order of the Sith Lords is an honorary body used to honor those who have earned the right to be called a Sith Lord and bear the title Lord or Darth. Membership in the Order can be achieved in several ways.
Dark Lord of the Sith
Grand Masters become Dark Lord of the Sith upon appointment and hold such status until retirement. The Dark Lord of the Sith is the leader of the Order of Sith Lords, it is from this status that he derives his position as leader of all three Lesser Orders and master of the Tripartite Path.
Sith Lord
Sith Lord is the standard status of members of the Order of Sith Lords. It is most commonly held by former Grand Masters who drop down to the status of Sith Lord after retiring as Dark Lord of the Sith when they leave the position of Grand Master. It can also be awarded to other members who the Grand Master deems worthy of recieving the Lord or Darth title.
Shadow Hand
Shadow Hand is the status of an apprentice of the Order of Sith Lords. It is most commonly held by the Deputy Grand Master as the "apprentice" of the Grand Master. "Shadow Hand" can be loosely considered the Sith variation of the title "Padawan", synonymous with the term "Jedi apprentice". Shadow Hands are not full members of the Order and so are not permitted to use the titles Lord or Darth.