Prima Luce
Prima Luce is the primary Dark Guild in this universe, Prima Luce is very secretive and no one even knows the members. However Prima Luce is very destructive and feared, as they often destroy towns leaving nothing behind only leaving 'Prima Luce' in the blood of corpses. Prima Luce is thought to have only a few strong Z-Rank Mages. The Magic Council have been trying to infiltrate it for years now, however with no postive results.
Knights of Lucerna
Knights of Lucerna is a dark guild comprised of banished knights from a country called Cerna. These Knights are very bloodthirsty and vengeful. The Knights of Lucerna often have no magical power inside them and depend on magical weapons. However the Council deems it weak and unimportant due to the small size of Knights of Lucerna. Although they have began recruiting in Fiore.
The Hive
The Hive is one of the more dangerous guilds, not due to the power of their members, but rather due to the large number of mages in The Hive. The Hive's Army outnumbers Fiore's army. Although they are not exactly powerful. The Hive plans to become a separate nation, and they are willing to go to war with all of Fiore in order to do it. There have been rumors of The Hive being Prima Luce's puppets. Although only time can tell.