Tartaros

Tartarus (冥府の門, タルタロス, Tarutarosu) is one of the three major Dark Guilds and a member of the Balam Alliance, alongside Oración Seis and Grimoire Heart.

Strength
Not much is known about Tartarus, but one of their members, the Devil Slayer Silver, was able to freeze the entire Sun Village, its giant occupants, and even the Fire Dragon Atlas Flame, alone.

Trivia

 * Tartarus, 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.