User blog comment:JohnnyN64/END/Natsu theories don't add up/@comment-24039250-20140729225951/@comment-24039250-20140807024046

Mard Geer and the Celestial King knew each other so it wouldn't surprise me if Zeref found the keys in the past and Mard Geer being one of his demons (maybe his first, possibly why he refers himself to the King of Hades, if not first then second, first being END) at the time was there. Also Zeref made Eclipse Spell through his dark arts and Celestial Spirit Magic so its not that surprising if he found the keys or fought many Celestial Mages or something.