Till the crack of dawn, Eric stayed up and read through the yin cultivation book. The exorcism was only explained in a few pages so he had to find them all by reading it through. It was not that complicated. The first and only rule was that an exorcist can't defeat a ghost or an evil entity that is more powerful than them without the use of specific yang-powered tools.
Technically speaking ghosts and evil entities are all powered by very strong yin energy. Yang energy which is the opposite of it can easily weaken them. That's why ghost boy would never come out in the direct sunlight.
But right now Eric did not have any yang-powered articles. He did not wish to spend any coin on the marketplace before he could confirm whether kai's girlfriend was indeed possessed or she was just being coerced by her family.