Grimoire Heart

Grimoire Heart Guild (グリモアハート Gurimoa Hāto 悪魔の心臓 lit. Demon's Heart) is one of the three Dark Guilds that is a part of the Balam Alliance. Its members are very interested in releasing Zeref. Currently, in the S-Class Trial Arc, they have detected Zeref's magic and are at Tenrou Island battling Fairy Tail members so that they can retrieve him.

Location and Buildings
Grimoire Heart doesn't seem to have any stable localization. Its members presumably live and travel on a giant airship. Because of this, it is an elusive guild.

This airship has heavily reinforced plating and is equipped with a version of the Magical Convergent Cannon - Jupiter. Despite its large size, it is quite fast and maneuverable, as it was seen dodging a punch from a gigantic Makarov.

Strength
As a member of the Balam Alliance, Grimoire Heart is a very powerful guild with highly talented mages. Examples include: Ultear, who was chosen to be selected as a member of Magic Council and who was even strong enough to destroy it and fool Jellal Fernandes, one of the Ten Wizard Saints, into believing that she was Zeref, and Hades, who was actually the master of Fairy Tail before Makarov and who defeated him without sustaining a single injury. Unlike the Oración Seis, Grimoire Heart has far more than six members, having several core members as well as numerous members of lesser rank, most of whom are dressed in hooded cloaks. Overall, it can be assumed that Grimoire Heart is the stronger of the two dark guilds.

Two of their scouting, lesser-rank members (Kawazu and Yomazu) were able to initially defeat Fairy Tail's Gajeel Redfox and Levy McGarden, both of whom were considered to be potential S-Class mages for Fairy Tail. This serves as an omen to the great power of their core members. This great power was finally demonstrated when Azuma, one of the core members, defeated Pantherlily, Wendy Marvell, Mest Gryder, and Charle all at once. Later on, Hades explains that the Seven Kin all use Lost Magic, and as such would never be defeated.

History
What is known of this guild is that its current guild master, Hades (real name Purehito), was formerly the second guild master of Fairy Tail, before passing the guild master title on to Makarov so that he could go on a journey. Hades later reveals that he spent years raising the Seven Kin of Purgatory and that they are his greatest mages who use magic that was as close as possible to the source of all magic: Lost Magic.

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. Each of these seven members possesses a Lost Magic.
 * Ultear
 * Zancrow
 * Caprico
 * Rustyrose
 * Kain Hikaru
 * Meredy
 * Azuma

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. Yomazu was still conscious, and was threatened by Erza to disclose his guild's goal, which he did so after he was severely beaten by her.
 * 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.
 * Numerous people (easily several hundred members) dressed in dark hooded cloaks. After these members were sent to the island, they removed their cloaks. Most of them appear to rely on physical ability (and may thus lack magic), as they were carrying swords, spears, hammers, staffs, etc. However, a rare few possess basic magic abilities, such as one member who foolishly launched a bullet of fire at Natsu. Most of these lesser-rank members were easily and quickly felled by the groups of Fairy Tail members.

Trivia

 * Grimoire is a textbook of magic.
 * Grimoire Heart is the only guild so far in the series that has a giant airship for its headquarters; this could be a reference to the Blue Guardians from Hiro Mashima's earlier work, Rave Master.