Diabolus

Diabolus is one of the four most powerful Dark Guilds in the world, and part of the Balam Alliance, alongside Oración Seis, Grimoire Heart and Tartaros.

Organisation
As most of the system is unknown, Diabolus seems to have a group known as "The Seven Sin Generals," or "Seven Sins" for short, with it´s Guild Master, Lord Beelzebub, being the leader of Dark Guild.

Strength
Although their full strength is unknown, one of it´s elites, Envy, was able to take down an entire town with ease, as well overpower Natsu Dragyama before awakening his powers.

Trivia

 * "Diabolus" is Spanish for "Devil."
 * The Guild were named before the reveal of the Legal Guild of the similar name in the 100 Years Quest, the Sequel to the Canon Story.
 * Among all the Dark Guilds of the Balam Alliance, Diabolus was the only one Grimoire Heart was in a good relationship, as well had helped each other in the past.