… is the only kingdom we know basically nothing about. Do we know anything about it aside from that it’s an outer kingdom that’s good with medicine, its dynasty is young yet, and that hanjyuu aren’t allowed in university?
No. I don’t think so. We don’t even know the ruler’s name! All the other kingdoms got some sort of an introduction, and even though we haven’t met him, Ryuu’s king has a name.
So I have a headcanon!
Shun is the kingdom of medicine, spies and assassins! It’s very secretive and keeps its gossip to itself. Gossip is a very valuable currency in Shun and therefore it has spies trying to weasel out every little morsel of it. And to keep the spies out, and to settle vendettas, people poison each other. Just think - medicine is just a different dosage/preparation of poison.
In order to save the ruler from getting poisoned, Shunki has a specific shirei to taste-test their food. Shun-ou is fairly pleased by this, because before ascending the throne, they risked poison with every meal they had.