User blog comment:Natsu11/Zeref and Natsu?/@comment-174.99.74.12-20130704030400/@comment-7418318-20130704092122

Considering that the Celestial Spirit King told Lucy that she's the first Human he invited to the Celestial Spirit Realm, Layla spending a long time in there is out of the question.

Gildart's never said anything about Lumen Histoire other than being surprised about it being below the Fairy Tail guildhall. He never said even a single word about it being dangerous or powerful or anything else.