Scarlets

Scarlets is the strongest and most powerful dark guild in Fiore. The dark guild itself is located in Magnolia, it's location is the south of Fairy Tail guild hall. To get to the Scarlets, you'll need to visit the forest. Scarlets has been the most threatening dark guild. It's founding master is unknown, but it's current master is known to be Lost Fernandes. The Scarlets have killed 20% of the world's population. It is known that Scarlets revived E.N.D. with the help of Siyami Kiba, they also unsealed Burakku, and they've damaged Fairy Tail by a scale of a large amount for the Scarlet Moon Plan.

Scarlet Moon Plan
The Scarlet Moon plan, was a plan to declare war against Fairy Tail. While this happened, Scarlets managed to pull out a forced full lunar eclipse, a blood moon. This plan to defeat Fairy Tail, was just a plan to defeat Fairy Tail, because they're the strongest legal guild in Fiore, the Scarlets only plan to defeat Fairy Tail so that there's nothing that can stop the apocalypse the Scarlets made.