Tartaros

The Tartaros Guild (タルタロス Tarutarosu 冥府の門 lit. Underworld Gate) is one of the three major Dark Guilds and a member of the Balam Alliance, alongside Oracion Seis and Grimoire Heart. Nothing else is known about them. Except for the fact that their master is actually Lucy Heartfilia's mother.

Trivia

 * Tartaros, in Greek mythology, is a deep, gloomy place, a pit, or an abyss used as a dungeon of torment and suffering that resides beneath the underworld.