Here the Skaven learned how to be ninjas.
They learnt to be ninjas in Cathay.
This is literally everything we know about Nippon.
There are two known characters from Nippon: The Red Ronin and Lady Katsui, who lives in Marienburg. According to End Times, they've got the best fleet ever, but the don't do shit with it. It's the best fucking fleet in the world. That's right, not druchii black arks or asur dragonships, fucking Yamamoto-Class Battleships.
There is also an autistic, weeaboo fan-made armybook for it with Oni, Lightning spewing dragons, ninjas and Samurai monks. The Norse armybook is better though, it would be even better if it weren't based on outdated, contradictory fluff.
Also, "Nippon" is literally the Japanese word for "Japan", because of course.
- Nippon Fandex 
|Regions and Areas of the Warhammer World|
|Areas of The Old World:||The Empire of Man - Bretonnia - Albion - Estalia - Tilea - Kislev - Norsca - Border Princes - Worlds Edge Mountains|
|Areas of The New World:||Naggaroth - Lustria|
|Areas of The Eastern Lands:||Cathay - Nippon - Ogre Kingdoms - Dark Lands - Kingdoms of Ind - Khuresh - Eastern Steppes|
|Areas of The Southlands:||Nehekhara - Araby - Badlands - Mashes of Madness|
|Other Areas of the world:||Ulthuan - Athel Loren - Chaos Wastes - Skavenblight - Lost Isles of Elithis|
|Main bodies of Water:||The Great Ocean - The Far Sea - The Sea of Dread - Inner Sea of Ulthuan|