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 comprise the Balam Alliance, as well as the strongest Dark Guild. Its members are currently battling Fairy Tail members at Tenrou Island.

Goals
When first shown in Chapter 131, Grimoire Heart's goal was to acquire all the keys that could unlock Zeref's seal; it only had one then. When they are seen again in Chapter 209, they had finished acquiring all the keys and were now aiming to locate him, physically capture him and bring him to Hades, and bring him out of his slumbering state. They have the intention of using him to rule a world (called "The Ultimate Magic World") where the 90% of the world's humans that cannot use magic will not be able to survive. Caprico later reveals that another one of Grimoire Heart's goals was to destroy Fairy Tail. Currently, in the S-Class Trial Arc, they have located Zeref at Fairy Tail's holy ground, Tenrou Island, and defeated his slumbering-state form, but have not brought him back to Hades or released him yet. They are currently winning most of their battles against Fairy Tail members.

Location and Buildings
Grimoire Heart doesn't seem to have any stable localization. Its members live and travel on a giant airship that flies high above the land. 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 one of the three most powerful dark guilds. In fact, it is stated to be the strongest dark guild by Levy 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. Indeed, many of its core members have been shown to be incredibly powerful, which is explained by the fact that they use Lost Magic. Below are some of the most impressive feats of Grimoire Heart members:
 * Ultear Milkovich: She had enough strength to be chosen as a member of the Magic Council, whose headquarters were later completely destroyed by her. For many years, she had fooled Jellal Fernandes, one of the Ten Wizard Saints, into believing that she was Zeref. She later defeated the sleeping-state Zeref in a short battle.


 * Yomazu and Kawazu: The two scouts of the guild held the upper hand for most of the battle against former S-Class mage Gajeel and S-Class candidate Levy. They eventually stalemated Gajeel after Levy went to retrieve help.
 * Azuma: He won two battles in quick succession against Fairy Tail members, both in which he was fighting multiple opponents at the same time. In the first battle, he defeated Pantherlily, Wendy, Charle, and Mest Gryder. In the second battle he defeated S-Class mage Mirajane and her sister, Lisanna, albeit both had been in previous battles. 
 * Hades: He defeated Makarov, a member of the Ten Wizard Saints and the current Guild Master of Fairy Tail, in battle without sustaining a single injury.
 * Zancrow: In his first confrontation against Natsu, he completely dominated their battle and blasted him far away. In his second confrontation with Natsu, he still had the upperhand until Makarov intervened in the battle on Natsu's behalf. Despite being heavily wounded into immobility, Zancrow retained consciousness while Natsu collapsed from his injuries.
 * Caprico: He is a Celestial Spirit who has been residing in the human world for 17 years, because he was possessed by Zoldeo after Layla Heartphilia passed him on to a new user. While in Caprico's body Zoldeo was capable of dominating four Fairy Tail mages in battle all at once (Lucy, Cana, Gray, Loke), two of whom were S-Class candidates, and forcing three of them to retreat. Caprico is now a spirit of Lucy after Zoldeo was defeated by the collective efforts of Loke and Caprico.
 * Rustyrose: He defeated S-Class candidate Elfman and his Raijinshuu partner Evergreen without sustaining a single injury, albeit both were battle worn from their previous battle with Mirajane.
 * Kain Hikaru: He defeated someone with as much physical strength as Taurus with a single palm strike, kicked back several of Sagittarius's arrows back at him, and withstood Scorpio's sand cannon and defeated him in a single palm strike. He also fought Natsu, Lucy, and Happy all at once and dominated most of the fight before being defeated.
 * Bluenote Stinger: Although his powers are unknown, he was able to defeat an entire Blue Dragon Regiment by himself and judging by his behaviour with Hades, he is probably much more powerful than the 7 Kin.

History
What is known of this guild is that its current guild master, Hades (real name Purehito), was the second guild master of Fairy Tail. However, 48 years ago, he bestowed the guild master title on to Makarov so that he could go on a journey. On this journey, Hades traced back the origins of magic to Zeref, where he saw what can be called 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. Hades later revealed that he spent years raising the Seven Kin of Purgatory and that they are his greatest mages who use magic that is as close as possible to the source of all magic: Lost Magic.

Guild Master

 * Hades

Top Member

 * Bluenote Stinger

The Seven Kin Of Purgatory
These are the strongest seven fighters of Grimoire Heart under Hades (with the exception of Bluenote). They are the main forces of the dark guild, the strongest formation of Grimoire Heart. Each of these seven members possesses at least one Lost Magic.
 * Ultear Milkovich
 * Zancrow
 * Caprico/Zoldeo (former defected, latter deceased)
 * Rustyrose
 * Kain Hikaru
 * Meredy
 * Azuma

Lesser Rank Members

 * Yomazu - A dog-like man dressed in samurai armor who wields a katana and who manifests various words that he writes in the air into powerful magical attacks.
 * Kawazu - A chicken-like man, complete with talons and feathers, who can rapidly bombard his enemies with eggs from his mouth that can produce fists.
 * 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.