Fairy Tail's Celestial Berserker (Not a story but an Idea)

What if one day after finishing another job Lucy happened to receive a one-of-a-kind Celestial Key that was black with red lines running through it and decided to open a contract with the key's spirit, how would future adventures turn out when a berserker is released. (A story of what if Fate's Berserker Lancelot was a celestial spirit that is contracted to Lucy at some point in the story)