When one of the two seals at the left side of the Chaos Sanctuary is unlocked, the Grand Vizier of Chaos emerges with a horde of Storm Casters. The Grand Vizier must be defeated before the player may engage Diablo.
The Grand Vizier is usually considered to be the least dangerous of the three Seal-guardians. Through maneuvering the player may be able to trap some of his minions behind the walls of the Sanctuary, allowing the player to defeat the Vizier more easily.
By some fan-beliefs, it is believed that the Grand Vizier of Chaos is Archbishop Lazarus from Diablo I. This holds some loose truth, as a "vizier" is another title to describe a head priest. Also what supports this is that the Grand Vizier of Chaos is one of the three guardians to unlocking Diablo, similar to Archbishop Lazarus' position in Diablo I.