Infiltrating the Black Temple: Follow Kanrethad and Jubeka's trail into the Black Temple by looking for her mark. After clicking on the mark on the front door, Warlocks can enter a special version of the Black Temple and complete the "Pursuing the Black Harvest" event.
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 annihilate 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.
... none of this is canon now. It was going to be a ton of work to tell the rest of this story and frankly, I don't know if even I could have pulled it all off.
I'm pretty happy with this ending, it's hard, it's epic and it develops the characters and uses all of the plot hits.
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. :)
Hotfix (2013-04-04):Fixed an issue where Warlocks remained in the wrong phase at The Black Temple after completing the quest Seek the Signal.