The hiding celestial key

This story is about juvia and gajeel going on a mission master macarov gave them , there are 12 celestial Keyes right ?
But what if there is a 13th one ? What is going to happen ? What's this mission ? Why nobody ever heard about this « key » ? Are they gonna succeed their mission ?( Lucy isn't part of the mission and no she won't take it ... ) there would be some gruvia and some gajuvia moment too :) u can consider the gajuvia moment as friendship moments or romantic moments it's as u like I don't judge ur opinions on ships . I'm sorry this summary is fucked up but I promise u the story is cool (:
Oh and most of the characters here aren't owned by me their from fairy tail but I will create new characters by my own if u read fairy tail you'll know what I'm talking about thanks and nice reading bebes <3