Fairy Heart

Eternity Magic: Fairy Heart (妖精の心臓, フェアリーハート, Fearī Hāto), also known as Lumen Histoire (光の神話, ルーメン・イストワール, Rūmen Isutowāru), is Fairy Tail's greatest secret and a very powerful Magic said to be superior to the "Three Great Fairy Magics."

Description
Fairy Heart is located in a secret underground room in Fairy Tail's original building. This secret is only known by Fairy Tail's Guild Master and is passed on to the new Master successively. Precht showed it to Makarov when he became Master. Years later, Makarov showed it to Gildarts Clive when he told him that he wanted him to be next Master of Fairy Tail. According to Ivan Dreyar, Fairy Heart is Fairy Tail's darkness, while according to Mavis Vermilion, it is Fairy Tail's light. Later, it is revealed that Fairy Heart is in fact the body of Mavis Vermilion encased in a crystal. Makarov refers to it as the "ultimate weapon" of Fairy Tail.

In X781, the Alvarez Empire attempted to mount an invasion of the continent of Ishgar with their intention being to capture Fairy Heart, but was stopped by the Magic Council's firing of Etherion. However, the empire never relinquished their intent to capture the artifact, and Makarov expected them to attempt invasion once again, especially with the Magic Council being crippled after the Massacre at Era and the Fairy Tail-Tartaros guild war, which left Etherion and Face rendered useless.

It is revealed to the guild in chapter 451 that Fairy Heart is in fact a source of infinite magic energy, stemming from a lingering residual magic source within Mavis due to the Ankhseram Curse.