Indus was a country to the Far East, between the the rest of the kingdoms, like Araluen and Skandia, and Nihon-Ja.
It was mentioned by George Carter in the The Emperor of Nihon-Ja that it is the Silisian Council's easternmost country. He went there from Nihon-Ja to contact Araluen after the warlord Arisaka rebelled against Emperor Shigeru.
The exact geography of Indus is unknown due to the fact that it is too far east to appear on the World map. However, it was known there was a river passing through it, and that it was very close to Nihon-Ja. Being in the east, it was probably farther east than the Eastern Steppes and the Middle Kingdom of Ursali. It is assumed Indus has a peace treaty with Nihon-Ja since they are both in the far east as well as Ursali because they are near. It also might have some sort of relation or peace treaty with Araluen
It is most likely based on Medieval India. Indus has only been visited by George Carter