Grimoire Heart

Grimoire Heart (グリモアハート Gurimoa Hāto 悪魔の心臓 lit. Demon's Heart) is one of the top three Dark Guilds that the Balam Alliance is comprised of, as well as the one commonly considered the strongest Dark Guild.

Goals
When they first appeared in the storyline, Grimoire Heart's goal was to acquire all the keys supposed to be used to unlock Zeref's seal, as well as to locate the Black Wizard himself. Once these goals had been achieved, their intention was to use him, discovered in a slumbering state on Tenrou Island, to rule a world (referred to as the Ultimate World of Magic) where the 90% of the world's humans, those that cannot use Magic, wouldn't be able to survive. Due to the Fairy Tail guild's presence on Tenrou Island when Grimoire Heart decided to make the move to unseal Zeref, one of its goals was also to get rid of "Fiore's #1 guild" in order to reach Zeref; a goal which, before finding all of the keys, they were also willing to leave to others, such as the fellow member of the Balam Alliance Oración Seis.

Location
Grimoire Heart doesn't seem to have any stable location. Its members live and travel on a giant airship that flies high above the land. Because of this, it is an elusive guild.

History
What is known of Grimoire Heart is that its founder and original Guild Master, Hades (real name Purehito), was the Second Guild Master of Fairy Tail, and was made such by Fairy Tail's founder, Mavis Vermilion. In year X736, he bestowed the title of Third on Makarov Dreyar so that he could go on a journey. On this journey, Hades traced back the origins of Magic to Zeref, and saw what he referred to as the "essence of Magic". It is assumed that he turned "evil" after this point, and that he created Grimoire Heart and became its Guild Master in order to awaken Zeref and make The Ultimate Magic World real. Hades proceeded to gather young boys and girls with strong Magic potential, such as Ultear Milkovich, and spent years raising and training them for them to become the Seven Kin of Purgatory, some of the greatest Mages under his command, who use Magic that is as close as possible to the source of all Magic, taught them by Hades himself: Lost Magic.

Strength
As a member of the Balam Alliance, Grimoire Heart is one of the three most powerful dark guilds. In fact, it was stated to be the strongest dark guild by Levy McGarden, and one of the three greatest protuberances of the Magic world (alongside Fairy Tail and Zeref) by Lahar. Unlike the Oración Seis, Grimoire Heart has far more than six members, having several core members as well as numerous members of lesser rank. Many of its core members have been shown to be incredibly powerful, and when they arrived to fight Fairy Tail at Tenrou Island they managed to battle and even defeat some of Fairy Tail's core members. Its Guild Master, Hades, was able to defeat the Ten Wizard Saints, Makarov Dreyar with ease. The deputy commander of the guild, Bluenote Stinger, was able to fight on almost equal grounds with Gildarts Clive, and every member of its strongest team (the Seven Kin of Purgatory) have proven themselves to be of S-Class level strength. Some of them even managed to combat powerful S-Class Mages such as Erza Scarlet on equal grounds. Although after their defeat by Fairy Tail, Grimoire Heart's strength lowered significantly, three of their members were killed including the Guild Master, and two (technically three) of their members defected the guild.