User blog comment:BlackWizardZeref/Fairy Tail: Chapter 436 Memoirs Review with a brief mentioning of Chapter 435 Victory Shout!/@comment-209.112.215.51-20150519051834/@comment-203.177.75.27-20150521083212

Maybe tartarus was formed when END aka Natsu was still in his demon formed, before Igneel sealed End in a book.