You are probably wondering who the Seiryu were, right? Well then, let me shed some light in here.
The Seiryu were a clan of powerful warriors and magicians. They have been around from even before the foundation of the land of Equestria. And, during it's foundation, many times they aided the regents of Equestria in battle and had supported them when needed. Some members of the Seiryu were known and recognized as some of the greatest warriors and magicians of Equestria. And it was considered an honor to be trained by one of them. So great was their reputation. However, the Seiryu didn't answered anyone. No. For them to fight for someone, that someone had to earn their respect. For the Seiryu took principles and honor very seriously. And so, in the times they were summoned, they answered to the Regents ONLY, which at that moment were Princess Celestia's and Princess Luna's parents, King Solaris and Queen Stellar Lumina. If someone like, say, a snobby noble, decided they wanted to command the Seiryu or drive them around, they would soon find out that they were biting more than they could chew. The Seiryu wouldn't harm them. No. Instead, they simply let the nobbles do whatever they wanted to and ignored them. After they tired, they would give all and every bribe or monetary gift the nobbles had tried to buy them with to the Regents. The Seiryu made a point to keep their motto. Which was, "Power is not for oneself. It's for the others. It is to be used to the good of everyone. Not to personal gains. It is to be used to protect the innocents. Not to judge and punish." And so, they used their power and wealth to help and protect the less fortunate. Many books and poems were written in their honor, narrating their stories and their deeds. However, if one were to search for history records and other documents about them or tried to find their village, they would find nothing. That would be so for two reasons. One, before Leon palaced himself in suspended animation, he placed every magical barriers and protective, deciving and comuflage spells that he knew to make it so that the village couldn't be located by any means. Not even the regents could find them, if they wanted to. And two, Nocturnal Shield and Satsuki, as well as their ancestors, had asked the regents that, if something were to happen to them, that every document about them should be erased. They asked this because evil doers or simply ambitious beings could try to use the contents and information in the documents to cause harm to the people. And the Seiryu would rather be forgotten than allow such tragedies to happen. And so, the Seiryu and the Regents had divided all the knowledge they had in books and other documents in half. Each side had kept one half and decided to place a special spell that was linked to every Seiryu that already exsisted and to every newborn. That is, so long as the regents knew of their exsistence. Thus, since Nocturnal Shield and Satsuki had kept Leon's existence a secret, the spell didn't apply to him. The effect the spell would have, was that if the Seiryu ceased to exsist, all and every document related to the Seiryu would automatically be destroyed. However, in the end, and unbestknown to the regenst, only half of the documents were destroyed. Because, while the regents spell wasn't linked to Leon, the clan's spell was. Thus, since there still was a Seiryu breathing, even if in animated suspension, the documents in the clan's vaults and library remained unscathed. However, since the regent at that time (Princess Celestia) didn't knew about that, she thought that the Seiryu clan no longer exsisted. It was indeed a very dark, tragic and sad day in the history of Equestria when the land lost the Seiryu clan.