Grimoire Heart

Grimoire Heart Guild is one of the three Dark Guilds that is a part of the Balam Alliance. They are very interested in releasing Zeref. Currently, in the S-Class Trial Arc, they have detected Zeref's magic and are going towards Tenrou Island on an airship. They also have scouting members already on the island who are battling Fairy Tail members.

Location and Buildings
Grimoire Heart doesn't seem to have any stable localization. They presumably live and travel on a giant airship.

Strength
As a member of the Balam Alliance, Grimoire Heart is a very powerful guild with highly talented mages. An example would be Urtear, who was chosen to be selected as a member of Magic Council and was even strong enough to destroy it and fool Jellal Fernandes, one of the Ten Wizard Saints, into believing that she was Zeref. Unlike the Oracion Seis, Grimoire Heart has far more than six members, having several core members as well as numerous members of lesser rank dressed in hooded cloaks. Overall, it can be assumed that Grimoire Heart is the stronger of the two; this is reinforced by Levy's statement that Grimoire Heart is the strongest dark guild.

Two of their scouting members (Kawazu and Yomazu) were able to initially defeat easily Fairy Tail's Gajeel and Levy McGarden, which serves as an omen of the great power soon to be demonstrated by their core members.

History
The history of this guild has not been disclosed as of yet.

Guild Master

 * Hades

The Seven Kin of Purgatory (Core Members)
These are the strongest seven fighters of Grimoire Heart under Hades and are the most feared as well. Even Yomazu, a mage who critically injured Gajeel, said he was nothing before them and stood no chance of victory against them. They are the main forces of the dark guild, the strongest formation of Grimoire Heart. Only six of the seven have been completely revealed as of yet.
 * Ultear - The mysterious daughter of Ur who formerly was a member of the Magic Council. Her primary magic allows her to manipulate the "time" of various objects.
 * Zancrow
 * Caprico
 * Rustyrose
 * Kain Hikaru
 * Meredy
 * Unknown male member

Lesser Rank Members

 * Yomazu - A dog-like man dressed in samurai armor who wields a katana and who uses sound-based magical attacks in conjuction with it. He and Kawazu arrived at Tenrou Island first, where they held the upper hand in a battle against Gajeel Redfox and Levy McGarden. After Levy retreated to retrieve help, Gajeel stalemated the both of them.
 * Kawazu - A chicken-like man, complete with talons and feathers, who can spit out eggs that can then bombard his enemies with fists. He and Yomazu arrived at Tenrou Island first, where they held the upper hand in a battle against Gajeel Redfox and Levy McGarden. After Levy retreated to retrieve help, Gajeel stalemated the both of them.

Trivia

 * Grimoire is a textbook of magic.
 * It is speculated that Mest Gryder is actually a member of Grimoire Heart, though it remains uncertain for now. He may actually be the unrevealed member of the Seven Kin of Purgatory.
 * Grimoire Heart is the only guild so far in the series that is based out of a giant Airship; this could be a reference to the Blue Guardians from Hiro Mashima's earlier work, Rave Master.