Fairy Heart (妖精の心臓 Fearī Hāto), also known as Lumen Histoire (光の神話 Rūmen Isutowāru), is Fairy Tail's greatest secret and a very powerful Magic, said to be superior to the "Three Great Fairy Magics."
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 Lacrima. 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.
Mavis later reveals to the whole guild that Fairy Heart is in fact a source of infinite Magic Power, stemming from a combination of residual Magic Power left in Mavis's body due to Ankhseram's Curse and decades of failed experiments by Precht in an attempt to resurrect her. Its power is claimed to be so great that Etherion could, hypothetically, be fired an infinite number of times if its power source were to become Fairy Heart. Interestingly, should Mavis' ethereal body come under any sort of harm, then Fairy Heart, as well as her body within, will also suffer damage. Due to her plan in order to take down Zeref, Mavis recruited Cana to use Fairy Glitter to destroy her Thought Projection form in order to free her true body that's inhabiting the crystal, which Cana after numerous attempts succeeds in doing so. It was later implied by Zeref that the power source, known as Fairy Heart, is actually Mavis' very humanly being.