A handover event must’ve taken place with the instatement of every Emperor. Only then could the new Emperor learn what the previous Emperor’s powers were. However, since Han Jingru killed the Emperor directly, there was no such event - the man had to discover this secret by himself.
The Imperial Archives was a forbidden area in Dragon Emperor Palace. No one was allowed access to it except the Emperor. That was why Han Jingru thought the previous Emperor’s secrets must’ve been hidden there.
Han Jingru was powerful enough to rule over the entire world, even without having to be imprisoned in Dragon Emperor Palace, but his power was not enough to deal with the Qilin. If he managed to incorporate the power the previous Emperor left behind to his own abilities, he would have a better chance against the familiar.
Even though he already had a plan for defeating the Qilin, he didn’t want to cast all his hopes on divine punishment. He wanted to give himself a way out in case that didn’t work out.
The Imperial Archives wasn’t incredibly big. Everything was in plain sight, and nothing was out of the ordinary. However, he believed there to be a secret chamber somewhere - he needed to slowly search for it.
After making a few rounds at the Imperial Archives, Han Jingru finally stopped in front of a mural.