This quest is unavailable to characters of Evil-alignment.
This quest may be obtained before completing the preceding quest in the timeline, Diplomatic Liaisons, once 30,000 faction has been obtained with the Ashen Order. Regardless, Shu Fang Qi will reccommend that you to speak with Zhao Wu once you finish Diplomatic Liaisons.
You can access the zone selection menu to choose The Cave of Knowledge by F-targeting (pressing the letter F key, or whatever you've changed to from the default) or mouse clicking while hovering your mouse over the door.
Also note that there are TWO doors inside of the cave; only one leads to the Cave of Knowledge.