User blog comment:Omega natsu2/Episode 192: Live on for Her, Review./@comment-24053170-20140726035822/@comment-4829046-20140727123842

Again, I bring up the possibility of Zeref creating the keys or at least being present at the time of the first CS mage or at least has seen various instances of CSK coming to Human world. Whatever the case, CS is interconnected with Zeref and by extension- the Demons of the Book.