The Grand Coven is the supreme command of all witchcraft. Without their sanction, I’m forbidden to practice!
The Grand Coven was the supreme command of witches, sanctioning witches in their use of magic. According to the Grand Coven there are three types of witches in the world, first the "borrowers" -- witches that get their power from demons, presumably in exchange for their souls. The second type, and rarest are the "naturals" those born with a natural ability for magic. Thirdly are "students" who are trained and mentored by Grand Coven approved witches.
The Grand Coven's power has become severely reduced to the brink of being decimated by the Men of Letters; who used their political influence to whip the church and public into a frenzy over witchcraft, and taking advantage by plundering their archives of spells and potions and storing them in their bunkers.
Rowena mentions how the Grand Coven cast her out and has prevented her from actively practicing magic.
Rowena begs Crowley to make a case to the Grand Coven, which she describes as the supreme command for all witches for her to practice magic freely again. Eventually he captures High Priestess Olivette for her to do as she will. Under torture, Olivette reveals that the Coven has been decimated and is "a pale relic of its past power." The Men of Letters used their political power to whip the church into a frenzy and many of the best members of the Coven were burned at the stake or forced into hiding. The Men of Letters then plundered their most powerful spells and potions and scattered them through warded bunkers across the world and in their weakened state the Coven wasn't able to get them back. Rowena loses her fear of the Coven and turns Olivette into a hamster and decides to search for the missing spells and potions for herself.
Rowena reveals that a Grand Coven witch named Nadya was murdered by the Men of Letters who took her codex that could decode the Book of the Damned. Sam begins a search for the codex, eventually finding it in the Werther Box.
With the Book of the Damned and Nadya's Codex in her possession, Rowena begins recruiting Grand Coven members to join her "Mega Coven." The first group, a trio of witches scoffs at her idea, believing she is more delusional than before, but for their rejection of her offer, Rowena performs an incineration spell that burns the trio into ashes from the inside out.
Next she appears with a duo in a restaurant, who are more receptive to her offer to join her Mega Coven. However, a demon assassin sent by Crowley interrupts, killing one of the witches as Rowena flees.