In a nutshell:
No hit PC game could be complete without at least one expansion pack and this one for Civilization IV is more generous than most with six new scenarios and the all important new warlord unit.
Offering a new chance to change the course of history the Warlords expansion has six new scenarios to play through from 350 BC to the adventures of Genghis Khan in 1206 AD. The most interesting new feature though is the new great person type called the warlord, with unique personalities for each race that greatly improve the abilities of your military units. Other new features include six new civilizations, ten new civilization leaders, the ability to create vassal states and three new wonders of the world to build or capture. There are also two new leader traits for your civilisations figurehead, new unique buildings and tons of new units, resources and improvements.
Most exciting moment:
Ever wondered what Churchill would do at the Battle of Troy? Nows your chance to find out as you play through battles from throughout history or just make up your own fantasy conflicts.
Since you ask:
Although a full list of warlord units has not yet been announced they are known to included Augustus Caesar, Ramesses III, Sir Winston Churchill and Joseph Stalin.
The bottom line:
Civilization IV expands with more features and more historic cameos than ever.
Chinese Unification (350 BC): Take control of one of Ancient China's kingdoms in an attempt to unify the country by either the pen or the sword.
Peloponnesian Wars (430 BC): Mighty Athens and Sparta struggle for dominance of Greece and surrounding territories.
Alexander's Conquests (330 BC): Alexander attempts to conquer the world to the south and east of Greece, taking on the Persians, Egyptians, and Indians.
Rise of Rome (300 BC): Battle as Rome, Carthage, Greece, Gaul, or Egypt for control of the Mediterranean.
Vikings (800 AD): Become ruler of the Vikings and send your men out on numerous raids against Europe. Victory is decided by how much loot can be acquired.
Genghis Khan (1206 AD): Relive the conquests of one of history's greatest military leaders and capture or if you must, destroy all of Asia.