Shin is a legendary Yoma and the currently most powerful entity aside Kagura, and the leader, possibly King/Queen, of all Yomas. It is Kagura's primary target and according to Sayuri, it is the entity responsible of every problem and disasters in Senran Kagura Universe, and a primary target also for most if not every shinobies that know it.
It is the oldest of the Yoma, and also known as the Original One. It's Yomas' heart and soul, and in theory it's destruction will eventually destroy instantly all of them, and stop it's effects on SK's world.
Shin appears under the form of a baby in a crib, still growing. Through, as it grows, the Earth in SK become unstable, to the point of flooding most land according SKEV, as SKEV's parallel dimension is a representation of the attack of Shin when it is fully grown. It is currently powerful enough to repel Daidouji when by some way she entered in its lair.
Few Shinobies know about Shin, as mostly only the most experienced of them like Sayuri or particuliarly powerful like Daidouji can tell about it.
Aside from it's power and hints about it's appearance, and the fact that it is the most powerful and the leader of all yomas, too little is known about Shin.
The only way to attract Shin, or any Yoma of a kind, is to lure it to the world's own space-time, with a shinobi barrier. Blood must be spilled within in order to attract it.
But the solution come with a huge problem, as nothing guarantees that the blood will attract only it, and if there is any proportionnality between the quantity of blood and the power of the Yoma, the blood required will be the equivalent of at least one Holocaust!