The 400 Year Plan

The 400 Year Plan is an event that was implemented during the X300's and completed on July 7th, X777.

Prologue
Over 400 years ago, while Zeref Dragneel was out gathering herbs, Zeref stumbled across the Fire Dragon King Igneel, who unlike many other Dragons at the time was not opposed to associating with humans, and after acquainting with one another, the Dragon agreed to teach his soon-to-be foster son Natsu Dragon Slayer Magic and raise him with the ability to harness it. Some time after their meeting Zeref met Anna Heartfilia, who later be renowned as the greatest Celestial Spirit Mage, and began to concoct a plan with her that would require her power, in collaboration with the pacifist Dragons and Anna's clan to defeat Acnologia; humanity's greatest threat. The plan was to send the Dragon Slayers with their foster parents sealed inside them to a period of time that was rich in Ethernano so that their souls, which were maimed by Acnologia, could properly heal. This coincided with Zeref's initial plan to have his younger brother become powerful enough to destroy him.

Plan
With the three forces combined minds put together, Anna opened the Eclipse Gate—one of Zeref's creations which when empowered by Celestial Spirit Magic, has the ability to send ones to the future—allowing the five chosen Dragon Slayers (Natsu, Gajeel, Wendy, Sting, and Rogue) to be sent 400 years into the present time. By having plan passed down through the Heartfilia clan, Layla Heartfilia opened another version of the gate with the help of contacting all the other 11 Zodiac key-holders to connect the two time periods on July 7th, X777, which in turn allowed the Dragons to replenish their health. However due to her servant, Grammi with her Aquarius's passed down key at the time, unreachable as she was in Alakitasia, Layla used her life force to cover for the last remaining key, which in concession greatly damaged her body.

Aftermath
As a result, Layla's already frail body was plagued with Magic Deficiency Disease; seven days later Grammi returned from her travels and was felt immense guilt as she could of prevented this from happening, thus returned Aquarius' key to Layla so that her daughter, Lucy, could one day more readily inherit it. Grammi went on to be assassinated by her fellow colleague Zoldeo and some time after this, Layla passed away from her illness.

In X784, during the war between Fairy Tail and one-third of the Balam Alliance, Tartaros, Igneel battled with Acnologia after releasing himself and the other pacifist Dragons from their foster children's bodies, enacting their plan. In the end, Igneel was killed after ripping the Black Dragon's arm off, with the other dragons' souls disappearing as they continue to induce the Magna Carta, watching over the humans for eternity.
 *  The fulfillment of the plan to the demise of Acnologia, at this time, has not occurred.