Cathay is a large empire of humans in the far eastern lands of the Warhammer World based on real-world Ancient China. In fact "Cathay" is the archaic English word for China. Cathay is constantly at war with the Hobgoblin Khanate to the east on the Eastern Steppes, and the Northmen to the north. Cathay is north of Khuresh, west of the Far Sea, east of the Mountains of Mourn and north-east of Ind.
Very little is known of the lands referred to as Grand Cathay since travel and trade between the Old World and Cathay is so dangerous.
"The capital and seat of the Dragon throne of Cathay is Wei-jin. The worship of Tzeentch is legal and widely practised in Cathay, albeit in less extreme forms than the worshiping practices of the Northmen. To the east of Cathay across the sea lies Nippon whom Cathay has often warred with. The Cathayans have built a giant wall called the Great Bastion to keep out incursions of Hung raiders from the northern Chaos Wastes. The Cathayans have fought many wars against the Dark Elves and their fleets, though little other contact with the more traditional Warhammer races is reported, save for traders traveling overland (along the silk road through the Dark Lands) from the Old World. Other locations mentioned in Cathay are the Mountains of Heaven, where giant animated statues of the Cathayan Gods guard temples, in much the same way as Ushabiti do in Nehekhara."
Whether this depiction of the land of Cathay is true or not has yet to be seen.
While little is known of Cathay's military capabilities, it is known that an army of (supposedly) 100,000 men were assembled to invade Lustria, the home of the Lizardmen. Out of these 100,000 men, only 1 man returned. It is unknown whether a follow-up campaign was initiated.