Vice Hierarch Zenya:
Shilen created these monsters. They belong not to this world but the world of delusion. Shilen sent them to search for power for her ressurrection. They are stronger and more dangerous than regular monsters, but they are focused on their search, and won't attack you first. However, if they or their friends nearby are attacked, they will attack you in return. Beware!.
One more thing. There are Succubus Disciples scattered around the Imperial Tomb. You have to be extra cautious around them.