Accepted and completed...hurrah. This was not quite as hard as the green fire quest, but I completed the Destruction Warlock Legion challenge (Tugar and Jormog) with 897i (see below, I wanted to make sure I did it below 900i) and got the appropriate achievement for it. Quest is "FelTotem's Fall".
As an advice for fellow locks, I'll leave my strategy. I used a pre-pot and ofc a flask and food. I had 4 interrupts available: This Mortal Coil (joke)(45sec), Spell Lock(24sec), Grimoire of Service-Felhunter(1.5min) and ofc Fear. I used fear mostly for the Earthquake cast but also sometimes for Felburst cast. Focus always on Jormog because of the Sonic Scream cast.
Don't forget to refocus when he comes out of hiding (/target Jormog /focus target /targetlasttarget /cast [target=Focus] Havoc)
Unending resolve comes handy when you have no interrupts available and it's too late to cast Fear so you know you're gonna get a Felburst right in your face.
Talents: Backdraft, Rev. Entropy, Mortail Coil, Soul Harvest, Demon Skin, Grimoire of Service, Soul Conduit.
So next in line is Demonology, Sigryn and its "The God-Queen's Fury" quest. Seems hard because it has 3 mobs to deal with and a bunch of valkyries aiming at you.