Xunfei.

The story of this small kingdom is a classic.
It has been under Xhuanghen's grip for almost a thousand of years, but for most part of it, it never fully surrendered or accepted its fate.
Despite a stormy history, the real trouble occurred about 200 years before the current date, when the King of Xunfei decided to rebel to the Empire and regain its independence.
The Emperor, at the time, was the father of TianSi, not someone known to history for his benevolence and virtues.
Xhuanghen's answer to the rebellion was immediate and brutal, culminating in a blood bath. Despite the hate and anger, though, the Emperor couldn't just wipe the whole kingdom and claim the land as his own. Because a selected number of citizen of Xunfei were, and are, the only alchemists in the world to be able to get red copper from the minerals it comes from. And since their identity is as secret as their technique, Xhuanghen couldn't risk to massacre them all.

The Emperor decided to send a trusted officer to Xunfei, acting as a steward, someone above the King himself, loyal to Xhuanghen only. With him was also sent a squadron of soldiers, whom, to this day, are still the only people in Xunfei allowed to be trained and possess weapons.
Among the various, repressive measures imposed by Xhuanghen to prevent the risk of more rebellions was the prohibition for Xunfei to have its own army and military organizations, including the most basic guards. Everything military was handed to the Xhuanghen soldiers on the kingdom.

Nowadays the situation was still unchanged, except from one detail: the latest king, who was different from any of his predecessors. Innovative, charismatic, brilliant. Not just he quickly gained strong favour from the people of Xunfei, he was also determined to fix the situation between the Empire and his nation.
Nothing that could bring any good to Steward Khan whom, for two centuries, basically ruled a country that was hostage in its own borders.
If King YiFeng had made it to meet with the new Emperor, there would be good chances that the hostilities between them would be solved and that Xunfei would retain some autonomy, making the Steward lose his privileges.

Khan's plan to prevent that eventuality was simple: quietly get rid of everyone with right to rule and then propose himself, to the Emperor, as new King for Xunfei, as he'd be the most suitable, loyal to Xhuanghen only.
He couldn't just kill them all in an obvious way, in case the Emperor would decide to investigate.
With the support of a very displeased and disfavoured concubine, Khan set the crown prince to be poisoned. Not a poison that would instantly kill, one that would keep damaging his health, like a disease, till causing his death.
Xunfei is much less advanced than Xhuanghen and its doctors would never be able to find out what was really happening to the Prince.
After YiFan fell sicker and sicker, the rest almost came forward on its own. YiFeng decided to head to Xhuanghen to literally beg for help saving his son. He was willing to share the secret of his people in return to that, even, but his ship, 'casually', never made it to Xhuanghen's shores. It mysteriously sank, with no survivors.
The news of the death of King YiFeng reached the Empire of course, but what the Steward never revealed was the reason why the king would be on that ship, deliberately making it look shady, as if YiFeng had some plot in mind.

At that point, all that Khan would have to do would be wait for the young Prince to die too, what he couldn't calculate was that the Queen was the Avatar of Jeisin. Her presence near the child was somehow healing him from the effects of the poison. When they realised that the Queen might have to do with the improvements with YiFan's health, they decided to keep her away from him, but she was stubborn and didn't stop trying to go to him until Khan decided to kill her. He didn't find it a big deal, he doubted that anyone in Xhuanghen would wonder about the fate of the Queen. He threw her body in the water, counting that sharks would make it disappear in hours.