User blog comment:Miskos3/Fairy Tail Episode 249: Tartaros arc: The Celestial King vs. The Underworld King, Review/@comment-180.191.130.34-20150906035140

It seems that the celestial spirit king and mardgeer known each other even before the tartaros arc. So it is possible that long time ago someone has also summoned the celestial spirit king, a wild guess would be layla. And if someone summoned the king they need to break a golden key but the keys are still complete until the tartaros arc so it is also possible that broken keys can be fixed or there is other way to summon the king.