The Grand Vizer was a position of power in many courts, the equivalent to a modern Minister. Usually it was a court official appointed by the sultan or emperor to act as their advisor, executive and tasked with running the most important aspects of the court and empire.
On the Discworld this post existed in both the Klatchian and Agatean Empire, often appointed by lazy rulers who could not be bothered with the effort of actually ruling themselves. This often led to the problem of the Vizier holding more actual power than the emperor.
Many Grand Viziers were a major health hazard for heroes, often having them flung to the snakes or tortured in cruel and unusual ways (The Iron Turtle or 'The thing with the cheese-grater). In nations without a Grand Vizier, the High Priest often played a similar role. Many men who held the position became highly unbalanced often plotting against their own ruler. Rincewind, who personally fell afoul of at least two viziers, held that the selection process for choosing such people appeared to be asking "are you a power-hungry scheming madman? Yes? Good, I shall make you my most trusted minister!"
At the end of Interesting Times Emperor Cohan wisely used a different question to chose his new grand vizier, asking the peaceful and humble Twoflower "do you know anything about being a grand vizier?" and upon being told "nothing at all" hiring him on the spot and leading to hope for the future.
Known Grand Viziers:
- Nine Turning Mirrors of the Agatean Empire
- Lord Hong of the Agatean Empire
- Abrim of Al Khali
- Twoflower new Vizier of the Agatean Empire.
There are also High Priests which rule on behalf of religion, rather than a court.