Raven Tail was a Dark Guild operating in Earth Land. As of year X791, such status has been revoked and Raven Tail has been acknowledged as a Legal Guild. Following the events of the third day of the X791 Grand Magic Games, Raven Tail was disbanded.
The whole concept of Raven Tail, its creation and even its name and symbol seem to stem from the hostility towards Fairy Tail, as Raven Tail's founder Master Ivan Dreyar was excommunicated by his own father Makarov Dreyar for unknown actions against Fairy Tail itself. In year X784, Ivan claims that his goal is to gather enough money to have a war against Fairy Tail. As of X791, Raven Tail's Master is still eager for such confrontation, claiming that the waiting is finally over, and his underlings started hostilities with the rival guild during the Grand Magic Games, attacking its members and picking on them during the competition's events. The reason behind Ivan's enmity towards Fairy Tail stems from his desire to obtain Lumen Histoire, a secret which his former guild treasures, and which is unknown even to most of its members.
Raven Tail's headquarters appears to be located in a rocky wasteland, being surrounded by large formations of bare rocks. The building itself is dug into the rocks, consisting of a high, slender and grossly carved fortress seemingly shaped out of a mountain, possessing jagged walls studded with rough openings of various sizes, acting as windows, and sporting slender spires protruding upwards from its entire perimeter. One of such spires, possessing a balcony with a stone balustrade, houses what looks like a laboratory of sort for Master Ivan Dreyar, packed full of vials of various sizes and books set out in bookshelves lined up the rocky walls, with disturbing ornaments such as skulls adorning the entire place.
As of year X791, the area around the fortress is mildly different in look, with some of the rock formations composing it sporting extremely elongated and sharp protrusions similar to stalagmites. It is also worth noting that the entire place is full of very large bones piled up everywhere, seemingly animal ones, and that the area is marked by a ragged light flag secured to a gnarled staff stuck in the rock, near to which sit a small saber, with its nicked blade similarly stuck into the ground, and a human skull; some proofs that, even if Raven Tail is currently a Legal Guild, it's not to be underestimated or lost sight of.
Raven Tail's headquarters received notable changes in the anime portrayal, where the wasteland surrounding its is ploughed by large chasms, with some pillars of dense smoke reaching up to the sky. The fortress itself is heavily altered in appearance, being notably smaller in width and being seemingly composed of massive corals, with a particularly high and slender central tower.
Ivan Dreyar founded Raven Tail some time after his father Makarov Dreyar excommunicated him from the Fairy Tail guild for unknown sins, several years prior to X784. During the seven years which followed Tenrou Island's temporary disappearance, Raven Tail lost its status as a Dark Guild through unknown means, being recognized by the Magic Council, and thus, as of year X791, being acknowledged as a Legal Guild. Prior to these facts, Makarov had Gajeel Redfox, a member of his guild, infiltrate Raven Tail as a double agent, pretending to support Fairy Tail's destruction and be Ivan's spy inside it, but instead monitoring the Dark Guild on Makarov's behalf. Even during the seven years of absence which followed the events of Tenrou Island, Fairy Tail's Master somehow managed to keep tabs on Raven Tail, thus being aware of every fact regarding the guild.
Raven Tail is a very powerful association. While the Magic Council still hadn't recognized it, it was seemingly the only Dark Guild not aligned with the Balam Alliance, managing to retain such status despite the lack of the nonaggression pact which safeguarded the guilds in the Alliance; a testament to its strength. After being granted the status of a Legal Guild and being thus given the right to take part in X791's Grand Magic Games, Raven Tail's team achieved the third place in the Games' preliminary event "Sky Labyrinth", beating a number of other teams from Fiore's top guilds. Some of the guild's members are shown to possess particularly powerful Magic: Obra has been described as being capable of reducing his opponent's Magic Power to zero in an instant, managing to do so from the distance, and Kurohebi employs Mimic, allowing him to copy other people's Magical abilities; a form of Magic which has been acknowledged as rare by Mavis Vermilion. It's also worth noting that Master Ivan Dreyar comes from a family of individuals known for their power, being the son of the Wizard Saint Makarov Dreyar, Fairy Tail's Master, and the father of Laxus Dreyar, a S-Class Mage who is considered to be Fairy Tail's strongest member after Gildarts Clive. During the Games, Ivan proved himself capable of producing illusions with extreme mastery, enough to fool the entire audience, which included powerful individuals such as Makarov, Mavis and the former member of the Magic Council, Yajima, with none of them managing to see through the false images he generated.
Due to their enmity with Fairy Tail, the very reason for their existence, Raven Tail has been described by Ivan as being a specialized "anti-Fairy Tail guild", with its members possessing Magic which the Mages of Fiore's former top guild are weak against. In spite of this statement, however, Laxus was shown to be fully capable of single-handedly taking on and defeating both Ivan and the guild's four strongest members (namely Obra, Flare Corona, Kurohebi and Nullpudding).