It’s quite complicated. The story started with Yag-Kosha and his group, when they tried to overthrow the rulers of their home planet, but failed and had to flee. They landed on planet earth, but then they were unable to leave.
After some time, the rulers sent a hunter to kill them (the Entity). He found and killed all of them, except Yag-Kosha, who trapped him inside the crater.
Several thousands years later, Yag-Kosha was imprisoned by one of his students, the mage Yara, who forced Yag-Kosha to build the Tower of the Elephant, and tortured him, to grasp his secrets. After some centuries of tortures, Conan sneaked into the tower and found Yag-Kosha, who begged Conan to kill him, and gave Conan the power to kill also the mage Yara.
When Yag-Kosha died, however, the prison who kept the Entity trapped, was weakened, or maybe even destroyed. This had several effects: the Entity could influence the thoughts of the people; there was also an explosion, that opened the crater, and released thousands of monsters; the power of magic was increased a lot.
The horde of the monsters was blocked on the swamps between Kara-Korum and Chosain. The infantry formed a front line, to stop the monsters, while the mages was killing them with their spells.
Following this, Yah-Chieng hired the Brittle Blade to kill the emperor Li-Yah, his concubines and children, then became Emperor. The first thing he did, was to organize an expedition into the crater, where he took the coffin of the Entity, which for some reason cannot exit from it, and brought it to Paikang.
Yah-Chieng then started to communicate only with the Entity, and secluded himself from the outside world, neglecting completely the Empire, which started to spiral into chaos.
One of the daughters of Li-Yah, however, had survived, and was raised by the Yellow priests of Yun. Their plan was to send assassins to Aquilonia, to lure Conan into coming to Khitai, and try to convince him to help them overthrow the Emperor. Conan however decides to send you, so the Yellow priests has to adapt their plans.
This is the point when you arrive to Khitai, and you have to find out what happens. After you have discovered all of this, you go to Paikang, to help the insurrection. There you will make contact with them, help them get troops from the various villages of the area, do some incursions in the city, to weaken the Emperor’s forces, then prepare to invade the citadel and kill the Emperor and the Entity. And that’s basically the end of the story.
This is quick summary of the story, I skipped some details and all the factions stories, but otherwise it would have been way too long. I hope it’s enough to give you a picture of the events.