Council of the Black Harvest

The Council of the Black Harvest was formed by six of the greatest warlocks of Azeroth. They gathered together after Deathwing's defeat to teach all warlocks advanced forms of magic stolen from their defeated foes. These powers would be shared with each other to further aid their kind against any future threats to Azeroth. They fought the Burning Legion alongside the other class orders during the demons' third invasion, and their order hall is the conquered Legion world of Dreadscar Rift. They are known as the takers of souls, commanders of demons, and masters of the dark arts.

Background
At first, the six warlocks refused to cooperate, bickering and tossing blame as easily as curses. Finally, after several nights, the human Kanrethad Ebonlocke spoke out about how the Alliance, the Horde, and various organizations around the world such as the Knights of the Ebon Blade had trained for war and kept getting stronger and stronger over time. He told them that they needed to do the same and that together, they would become more powerful than they would have ever been alone.

After much debate, it was decided that each warlock would be partnered with another in order to obtain new powers, that would then be shared with the rest of the Council. Although not every warlock in the Council liked each other, they knew that what they would be doing would greatly benefit them all. To make sure greed did not set in with individual members, it was agreed that any warlock that broke the contract, failed to return, or returned alone, would be struck down by the rest of the Council and their soul would be banished forever.
 * Ritssyn Flamescowl and Zinnin Smythe were sent to Sulfuron Keep to gather the powers left behind by the Firelord.
 * Shinfel Blightsworn and Zelifrax Wobblepox hunted down the remaining members of the Twilight's Hammer and Cho'gall's followers to persuade them to reveal anything they learned.
 * Jubeka Shadowbreaker and Kanrethad Ebonlocke returned to Outland. While Jubeka learned a lot about different races of demons, Kanrethad sought to put into practice what he had learned about Illidan Stormrage's demonic transformation, and succeeded in becoming a demon himself. This however culminated in a battle at the summit of the Black Temple between a warlock hero and Kanrethad, ending in the latter's permanent banishment by Jubeka.

Shadowlands
After the Fourth War, the Council of the Black Harvest makes its appearance in the warlock section of the Training Hall in Stormwind and the Barracks in Orgrimmar, with Laphandrus Voidheart, Evelyn Thorn, and Kazak Darkscream as warlock trainers.

Council
Each of the original members witnessed one of the great threats to Azeroth.

* The First of the Black Harvest chose his Sixth between Kira Iresoul and Lulubelle Fizzlebang.

Troops
They deploy Packs of Imps, Swarm of Wild Imps, Black Harvest Acolytes, Black Harvest Invokers, and Felguard Brutes.


 * Lowly among demons, imps are often underestimated due to their singular lack of toughness. Yet when in a group, they quickly replace the fallen, and their real threat is in how difficult it is to kill every single one of them.


 * The Acolytes of the Black Harvest are ready to sacrifice everything for the forbidden knowledge they pursue, including each other. Many of the newer acolytes are often found dead after experimental rituals.


 * It is rare for Acolytes of the Black Harvest to reach the rank of Invoker, as they are often consumed by the dark energies they call upon as part of their ascent.


 * Massive brutes which thrive on destruction, many believe them to be mindless, but they actually are quite intellectual, for a demon.

Original plans for the Council's storyline in MoP
Former WoW game designer Alexander Brazie, also known as Xelnath, shared the original plans for the quests depicting the adventures of the members of the Council of the Black Harvest during the events of Mists of Pandaria:

''Shinfel and Zinnin go into Ahn'Qiraj to investigate the old gods after torturing some twilight dudes to death. Zinnin brings a bunch of Kobolds in explicably. They discover Curse of Enfeeblement, learn Harvest Life from a dying Qiraji. Zinnin attempts to learn a more powerful curse, but goes insane. Shinfel is forced to drag him back to the council to survive.

''The Orc and the Worgen go into firelands, build respect for each other after fighting off lava spiders. They literally rip runes off of the walls in Sulfuron, and discover a way to pull lava directly from the well Ragnaros created. This power became cataclysm.''

The Orc and the Worgen work together to empower Chaos Bolt, and nearly annhilate each other, then discover the power of Ember Tapping to rapidly cauterize their wounds from an imp.

''The council meets up together, share their powers with the Warlocks of the world. Kanrethad takes the knowledge gained, calls the council a success and proposes a master plan once the Ata'mal crystal was purified in the arcane well of Black Temple. He awaits its completion.''

''Player brings the crystal, transfers too much power into Kanrethad. He loses control, summons a Pit Lord and passes out. While fighting the pit lord, you're overwhelmed, when the Ata'mal crystal fuses with your being, allowing you to control the Fel Energies of the Pit Lord without being completely corrupted. You use that power to defeat the pit lord, Kanrethad recovers, horribly wounded and acknowledges your mastery.''

''He questions his ability to handle the power alone. Summons the council and uses you, rather than him, as the focus spell to open the portal to Xerrath. Inside, you lead the council in daily quests to drive the legion out and conquer the island.''

[...]

''My hope was to create a group of Warlocks who are power-hungry, driven and powerful, but aren't completely consumed by that power... for once. Maybe only Player Warlocks can be that kind of character.''

Notes and trivia

 * The Council of the Black Harvest dealt terrible blows to the Burning Legion throughout the third invasion of the Burning Legion. They killed one of Mephistroth's most powerful lieutenants, Jagganoth, took control of his world of Dreadscar Rift, submitted the armies he had stationed there, stole the powerful artifact known as the Skull of the Man'ari from him, and enslaved his precious and mighty Eredar Twins. They also ruined 's ritual by stealing the Scepter of Sargeras that he planned to use along the Eye of Dalaran and Book of Medivh to open countless portals to all Legion worlds.
 * The Council's archives in Dalaran contain an immeasurable amount of magical knowledge that has been collected from various sources over time.
 * The Acolytes of the Black Harvest are ready to sacrifice everything for the forbidden knowledge they pursue. Many of the newer acolytes are often found dead after experimental rituals.
 * Three of the six Black Harvest champions in Legion have different specializations than what they were presented with in Mists of Pandaria. Zinnin is Affliction despite his interest in the destructive powers of Ragnaros and Sulfuron Spire, and Jubeka and Kanrethad are respectively Destruction and Affliction despite their research in Demonology and demons in Outland.
 * During 5.1 PTR, they were known as the Council of Six Daggers. Each member had a quote in the warlock talents. Wrathion had become concerned in their interest for his father's power. This, however, did not make it to live servers, and Wrathion is thus not involved with the Council.
 * The title of the order leader was originally going to be "Overlord". It was changed to "Netherlord" during alpha.
 * The Council was created with the intention of memorializing six long-time warlock players who helped Alexander Brazie overhaul the warlock class for Mists of Pandaria.