Earth Ghosts, Water Ghosts, Dream Ghosts are all beings beyond the Sixth Level, yet not reaching the Seventh Level. According to the Ghost Vendor, Earth Ghosts were once controlled by humans, attempting to break through to the Ghost God Realm and failing, leaving behind ghosts in the depths of the Hidden Realm.
Wherever there is water, Water Ghosts will appear. Those who've long been haunted by nightmares will attract Dream Ghosts. In the forest, if someone prays, it can also bring forth the shen kan of an Earth Ghost.
Zuo Jin and the Class Leader are both at the threshold of entering the Seventh Level. In terms of power, the Earth Ghost should be comparable.
Of course, Sang Que is referring to the true form of an Earth Ghost, not those Ghost Slaves sent out to collect debts. Back in Black Mountain Village, the Earth Ghost entangling the Li Family Widow was a Ghost Slave, not the true form.
