The title of Warlord was bestowed by Emperor Palpatine to his most respected and important warriors. Thrawn received the title when he was promoted to the rank of Grand Admiral, and presumably, the same was done for his twelve counterparts and Darth Vader. What responsibilities the title came with are unknown, though it is assumed that it came with many privileges and generally outrank any other official in the military and most officials within the government, with the exception of possibly the Grand Vizier and Executor, and, naturally, the Emperor and his appointed lieutenants and Dark Side Elite.
The title of Warlord was continued by Bacharan Valak during his reign, and he was known to have appointed many to the office, such as Jarl Rellik. Because of his militaristic favor, and his wariness of political machinations, he treated the position of Warlord as Palpatine had Grand Moffs. By 13 ABY, the only known Warlord was Darth Malign, appointed by Aleister Vadim. In 13 ABY, the Warlord assumed control over the Empire with the enigmatic Vadim ruling from the shadows. In 14 ABY a new Warlord was appointed, Vassily Korolov, had used his connection with the new Emperor to solidify the power base and close the remaining wounds between the military and government.
However, in recent times, the Warlord has been absent from all official functions. His office still issues edicts and his staff continues about as if nothing was wrong, but the reappearance of Malign, and Korolov's disappearances, lead many to wonder if the former Royal Guardsmen had finally met his destiny or was the victim of the latest round of Imperial Court Intrigue.
"Warlord" was also used informally to describe the leaders of the innumerable breakaway states that splintered from the Empire following the death of the Emperor. Some even used it as their formal title, presumably to imply a false connection to Palpatine's inner circle.