Don't lie to me like that. You want it. You want to do the end of this questline, because it's so badass. I was spinning circles in my seat and giggling maniacally at how Kanrethad completely wiped the roof of the Black Temple with my body for two hours straight, because it was so much fun to be challenged not merely as a player, but as a whole warlock. You're going to use abilities you've almost never used before in this questline, and you're going to love all of it.
I hope I don't have to say it too much, but spoilers ahead.
Codex of Xerrath
The first step is acquiring the Sealed Tome of the Lost Legion. Blizzard delivered on making an awesome questline that means something, so a rare spawn will be completely worth your time to chase after.
Currently, on the PTR, you loot the item from a Green Flame Infernal, a plain ol' infernal modeled mob that spawns right next to the Shado-Pan Garrison in Townlong Steppes. I highly doubt this is the final iteration, unless there are some highly unique infernals who like Demonic epic poetry and long walks on the Townlong cliffs.
The item instructs you to create a healthstone to combine it into the Codex of Xerrath, which will instruct you to seek advice. From whom, you ask; why, your minions, of course! The felhunter will not talk to you, and you should know by now that the imp is all babble and no brains, so ask a voidwalker, a succubus, or a felguard (or any of the Supremacy minions).
From there, you'll be directed to your capital city's warlock trainers, whether that's to see Sandahl in Stormwind or Neeru Fireblade in Orgrimmar. Each trainer will ask you to read over the Legacy of the Masters (Part 1) and you'll receive Jubeka's Journal to read.
You'll start in Hellfire Peninsula with Felspark Ravine, just south of "The Legion Front" text on your map. As you approach the area where the soulstone fragment is found, you'll get a Cold debuff aura, soon progressing to Warm and finally Hot. The soulstone fragment is a helm-sized purble gem that glows on the ground.
Stay for a bit in the Ruins of Farahlon in Netherstorm after you pick up the soulstone: you'll see a ghostly roleplay with Jubeka and Kanrethad learning about the observers. A similar roleplay will happen in Blade's Edge Mountains at Vim'gol's Circle just north of Forge Camp: Anger, where Jubeka and Kanrethad will learn a piece of demonic history before the Legion's influence. Finally, you'll end up at the Altar of Damnation near the Hand of Gul'dan mountain in Shadowmoon Valley, where you'll learn that Kanrethad didn't luckily pair himself with Jubeka in the Legacy of the Masters.
Back to Black Temple
I was enjoying the lore from Jubeka's Journal so far, but I laughed so very evilly in the beginning of Black Temple. Akama surprises you with a false document for you to pick up, but then you trick him back by blathering on about your faction sending you to investigate the Black Harvest. And, because you're trusted by the Ashtongue, the old fool believes you! Ha!
Little does he know what you want for yourself. But first, you must find Kanrethad and the rest of the Council of the Black Harvest. The first step is looking at what's up in the Reliquary of Souls. Listen to Akama.
Let me repeat that: listen to Akama.
This quest is a-maze-ing. If you charge headlong into the hallway leading to the Reliquary of Souls, you'll die a quick and foolish death (like I did, hmmm). Listen to Akama and use a lesser demonic minion to guide your way there.
After the Essence of Order is defeated, you'll fight through some demons to find Akama. Akama will race to the top of the Black Temple through a special door. You, on the other hand, will run the long way, picking up pieces of treasure for "safe keeping" as you go.
This part of the quest is currently bugged on the PTR, where it will fail the scenario stage if you do not reach the chest on Mother Shahraz's platform before the 5-minute timer is up. In a later build, you will simply be unable to pick up more treasure after the timer expires, but you'll still be able to run to the end to start the final stage. For now, focus on getting to the chest in 5 minutes.
For fel's sake, the opening seconds of the fight require you to enslave a Pit Lord -- yes, you read that correctly, a Pit Lord as your minion. Should you survive the onslaught of 60 sarcastic imps while dodging Kanrethad's one-shotting Chaos Bolts, you must struggle against magic-hungry felhunters to keep your pit lord under control. The datamined new warlock spell Cataclysm didn't completely disappear after all -- Kanrethad kept it. After that, I hear there's a doomguard or an eredar that shows up, but I'll have to keep up my attempts to find out.
Jubeka will assist you in banishing the corrupted Kanrethad, and you'll receive green fire for your role in this epic questline, along with a dialogue option to keep your fire orange if you so wish. Even if you want to stick around with orange fire, go face off against Kanrethad in this epic, awesome questline. The experience alone is completely worth it.