Grand Vizier was the office in the Galactic Empire responsible for maintaining the day-to-day operations of the Empire and the Bureaucracy. The Grand Vizier served as the Head of Government of the Galactic Empire from its foundation in 19 BBY until its fragmentation in 4 ABY. There was no Head of Government throughout the Post-Endor Imperial power struggle, as the Interim Ruling Council served as the all-encompassing ruling body over the remains of the Empire.
The Grand Vizier was the highest rank in the Empire besides the Emperor and the Executor in pre-Endor Imperial society. He oversaw the myriad departments, bureaus and offices and acted as personal assistant to the Emperor. Essentially, the Grand Vizier performed almost all the day-to-day duties which the Emperor couldn't perform due to the hunt for the remaining Jedi and the Rebellion. He acted as an intermediary for the Emperor; few spoke to His Majesty without first going through the vizier. One of his more loathsome functions was to act as official food taster for the Emperor. As Head of Government, the Grand Vizier stood at the top of the Imperial Ruling Council, acting as its top official.
The position of Grand Vizier was formed after the establishment of the Empire, and may have been a replacement for two preexisting offices, those of Vice Chair and Senior Administrative Aide. The first individual to serve in the office was Sate Pestage.
When Bacharan Valak was coronated in 8 ABY and the Imperial Civil War ended with the reunification of the Empire, the post of Grand Vizier was once more filled and resumed its Head of Government duties. However, the aftermath of the War of the Throne and the rise of Darth Malign effectively neutralized the Grand Vizier position temporarily, and the Warlord assumed the role of Head of Government until 15 ABY, when the Empire was restructured under Aleister Vadim and the Grand Vizier was brought back, with Warlord Vassily Korolov as its holder.
However, the Galactic Empire was set to crumble after Vadim was killed and the Galactic Civil War came to an end in 19 ABY. The newly proclaimed Emperor, Darth Maelstrom, chose Korynn Fleming to become the new Grand Vizier of the Imperial Remnant, and charged him with restructuring the Empire's various intelligence and bureaucratic organizations.
Fleming effectively (and in some cases, violently) wrangled the various organizations into one umbrella named, simply, Imperial Intelligence. He answered to the Emperor alone.