Legendary Quest

http://www.wowhead.com/talent#IburRGcGckI0r

I decided to not take soul swap because glyphed, it will not always be available, and Thyrinar NEEDS to have near 100% uptime on dots, it's very hard to heal yourself enough when you have to spend 3gcds dotting him up.

Haunt is important to keep up, but not as important as having 3 dots for maximum drain life effect.

I keybound devour magic, spell lock, coe, and pet attack target to my mouse, but if you can't do that, know that having these spells keybound is a gamebreaker. you NEED to be able to coe the adds and get your pet tanking them, instantly, while on the move. use your instant dots (agony and corruption) and if you find yourself out of range of the boss, apply UA and drain life spam the add till it is dead. it takes very little time for it to die with only 2 dots up though.

You'll need to get used to the frequency that he applys his healling buff. It's important that if you think he is about to do it to not move your pet to an add before stealing it.

This fight IS buggey, sometimes the pet dies in about 2 shots from adds, sometimes he glitches out in the middle. The key to him not glitching is to always make sure he is in range whenever trying to spell lock or devour magic. Make sure you are regularly silincing the boss, in order to get a little catch up time for healing. the best time is right after a beam, but before the next add. 

don't worry too much about shadowbolting, they take too long and drain life filler is still pretty good dps. Also, try to fel flame as much as possible whenever you are moving, this can end up being as high as 7% of your damage, and you won't have to waste globals getting more dots up for better drain life healing.

I used fel armor (it seemed to have better healing overall, and more useful healing, than demon armor), flask, int food, and dark intent on my pet with soul link up. i didn't glyph soul link, but that would have most likly helped, 

Try to use your guardians as an extra defensive cooldown if you get a little behind on adds, although, you're most likly screwed if you have 2 adds up at a time.

I found myself not needing to fear when things were going right. coe kiting and pet tanking worked very very well. However, it only worked when it worked. I felt like I had to get lucky with my pet wanting to stay alive or not. I'm not sure if it's bug related (didn't notice if he had immunity or resistance to the fire, but i don't think he did) or just luck, but some attempts my pet was a rockstar and others he was worthless.

When you get the haste buff start drain souling, but only after you get yourself to around 75% health, and don't rush things and lose control of the adds. I died at 160k hp because I got excited and it took me another hour to down him. 

Mainly, the game breakers are keybinds. The damage comes out SOOO FAST that you have ZERO time to be messing around. everything you do needs to be done yesterday. having easy and fast keybinds for key abilities, (the most important ones are pet attack, coe, devour, and spell lock) will dramatically improve your chances at winning this fight. when i was able to coe, pet attack, corruption, bane of agony, with zero gap in gcds, the fight totally opened up for me and i was able to get enough damage and healing done that I could manage the fight.

Also, try to always start on one side of the room and t-port to the other and just wait a bit for the breath. since it doesn't follow your path, it chases you directly, it's very important you put it exactly where you want it, getting cut off from adds or the boss can be very costly. i generally put all the fire on the front of the platform, kiting back and forth.

I think that's basically it, keep 3 dots up at all times, drain life filler, fel flame on the move as much as possible, pet tank and instant dot the adds with coe. remove buff, silence when you need just a little catch up. OH, MAKE SURE YOU ARE USING SHADOW WARD ON COOLDOWN this will save you a lot of damage and can quickly become a gamebreaker when you get a little behind on your healing, which you will at least a couple times during the fight.

If you're having trouble with the first part of the encounter, kill dogs before big guys. demo is best for the first half. wait to kill the 3 dogs BEFORE going up the gauntlet ramp. meta immo aura, hellfire, felstorm up the ramp makes it trivial. fear interrupt the huge shadow novas, and use the orbs on the big platforms. kill the dogs on the ground before switching to the big platform and going down. But honestly, anybody can learn the first half in about 2-3 wipes.

1. I did respec to 31/10/0. I know some probably did this successfully with a standard affliction raid spec, I just couldn't keep my pet alive w/o points in Fel Synergy that would come in a more raid standard 32/3/6 affliction spec. Plus Demonic Aegis really helped a great deal. I also glyphed Bane of Agony and Felhunter for extra damage and heals as I was not using shadowbolts and instead opted for a Drain Life filler to stay alive. I did NOT use Demon Armor as I found that I got as good (or better) healing from using Fel Armor and keeping my dots up.
2. I did switch to my Demo spec at one point. Just after I killed the three dogs (as affliction: abuse Soul Swap liberally on this quest btw) after being ice-blocked. By switching to demo here, I just Metamorphosis/ Immo Aura / Hellfired my way up the ramp with little to no difficulty with adds. I've also seen this done just staying affliction using Fel Flame. The hellfire route was simple.
3. Killing the dog when Taregosa does her 'breathe fire on the icewall' thing = simple. Just beat the hell out of the doggy and move when he starts to cast his spellflame or whatever. Simple basic mechanic and it shouldn't really cause trouble.
4. Also simple was the platform room. Just use Drain Life liberally as a filler and when killing the big adds, make sure to click the orbs around the platform to give yourself a shield. However, even if you don't do this to avoid shadow nova damage you'll probably be okay as you can soak one of these without dying...then it just becomes a matter of dpsing them down prior to a second shadow nova. A final note here...once you're on the big platform, be sure you kill the little adds that pop up from below and don't accidentally engage a big add on another big platform. You can survive if you do this (easily actually) but it doesn't help you get through any faster and it will take a chunk out of your health.
5. Thyrinar. Last fight was the tough one for me. As noted I used a drain life filler in addition to my dots (and I did use Bane of Agony for easy soul swapping).
   1. First thing I did was (as noted above) put my Felhound on passive.
   2. Set Thyrinar as focus and use one of the focus macros noted above. For simplicity I actually macro'd my Shadow Ward (to ensure a high uptime for mitigating blast damage) and Devour Magic (for purging the restorative buff) effects together on a single button. I also made a spell lock macro for interrupting twilight blast. They looked something like this:

   #showtooltip Devour Magic
   /cast [@focus] Devour Magic
   /cast Shadow Ward

   #showtooltip Spell Lock
   /cast [@focus] Spell Lock

   Remember: for these to work you've got to get up close to the boss (30 yard range of the felhunter). So, start close and keep an eye on your felpup's range from the boss.

   3. Adds kept killing me. So, into my standard Haunt, BoA, Corruption, Unstable Affliction, Drain Life and Soul Swap, Macroed in the following petattack command (as this made my pet automatically attack the add as soon as it was targetable. Even if I hadn't gotten my dots up on it yet.

   #showtooltip Bane of Agony
   /petattack [target=Twilight Invader]
   /cast Bane of Agony

   Because I was 2/2 in Fel Synergy my pet was quite durable and could take the beating associated with tanking adds. This allowed me to cast a quick soul swap to get my dots up on the add and then switch back to the boss. This step was probably the most integral to my success on this fight. With my Felpuppy tanking the add I was able to very easily kill Thyrinar on my first try with this new strategy. I didn't even need to fear or shadowflame an add at all. Between my dots and my dog adds died easily.