Clearly, the cultivation of the Ghost Head Infant is much higher than Lu Zheng's. With its aura being the basis for spell casting, it's impossible to see its true form.
This situation is different from when they were in Weizhou.
At that time, the Profound Heart Demon remained hidden, only revealing a mortal aura. If Lu Zheng had observed it with the Treasure Mirror then and remained calm, he wouldn't have been able to discern the Profound Heart Demon's details.
Unfortunately, when the Profound Heart Demon realized it was being observed, it panicked and exposed itself instead.
But this time, evidently, the aura of the Ghost Head Infant is much deeper than the aura that spell casting seeks, so the Treasure Mirror cannot display the true image of the Ghost Head Infant.
"An expert!"
With Lu Zheng's current level of cultivation, any opponent whose true nature he cannot discern must have at least two thousand years of Dao Practice.
