The 400 Year Plan is an event that was implemented in the late X300's and completed on July 7, X777.
Prologue
Zeref and Anna converse of their plan
Over 400 years ago, while Zeref Dragneel was gathering herbs, happening to stumble across the Fire Dragon King Igneel, who, unlike many other Dragons, was not opposed to associating with humans at the time. 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, though, Zeref met Anna Heartfilia 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 is rich in Ethernano so that their souls, which were maimed by Acnologia, could properly heal. This, however, coincided with Zeref's initial plan to have his younger brother become powerful enough to destroy him.[1]
Plan
 Layla opening the gate on July 7 |
With the three forces joined together, Anna opened the Eclipse Gate, one of Zeref's creations which, when empowered by Celestial Spirit Magic, has the ability to send beings 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 the 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 7, X777, which in turn allowed the Dragons to replenish their health. However, as Grammi, who was in possession of Aquarius' key, was unreachable in Alakitasia at that time, Layla used her life force to cover for the last remaining key, which, in concession, greatly damaged her body.[2]
Aftermath
Layla ill from loss of Magic
As a result, Layla's already frail body was plagued with Magic Deficiency Disease. Seven days later, Grammi returned from her travels and was feeling immense guilt as she could have prevented her sickness from happening, thus returning 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.[3]
In X791, 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 childrens' bodies, enacting their plan.[4] 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.[5]