Felmyst is the third boss of Sunwell Plateau, accessible after Brutallus is killed. Felmyst is the remains of Madrigosa, a blue dragon that attempted to subdue Brutallus before she was summarily slaughtered. The blood of the pit lord seeped into the soil and corrupted what was left of the noble dragon, warping her into a fel dragon and granting her new and dangerous abilities.
Why are we talking about this boss?
Unlike the preceeding boss, Brutallus, Felmyst requires a lot of movement and
situational awareness. Felmyst will frequently alternate between "ground" and "air" phases, much like Nightbane from Karazhan. Also like Nightbane, the air phase is characterized by a horde of skeletons raining from the heavens to slaughter all of the living. That, plus all the raid damage being thrown around, means you need to be light on your feet and keep your wits about you. One false move, an inopportune lag spike, or any hesitation in your movement can spell fatality for you and yours.
What to do when facing Felmyst
The fight is almost squarely on the healers (notably the priests), but there are still a few things you, as a ret paladin, about which you need to be aware.
Felmyst is about awareness. First, ground phase awareness. Ol' Mystie will periodically cast Gas Cloud, which is a debuff that drains health and mana from everyone in range and can only be dispelled via immunities or Mass Dispel. You need to stay aware of your HP - if you drop low, gulp a health potion or healthstone. The amount of raid damage in this fight will keep your mana topped via Spiritual Attunement gains from heals, so don't save the potion cooldown for a mana potion. Keep yourself alive, the healers are going to be stretched.
The second ability during ground phase that you need to be aware of is Encapsulate. Felmyst will turn and target a random person in the raid other than the main tank and cast this ability on that raider, lifting them in the air and dealing damage to that person and everyone within 20 yards. You need to carefully watch your surroundings, and at the first sign of Felmyst casting Encapsulate, run like your life depends on it. Because it does. Before Arcane resistance is figured in, Encapsulate can deal about 10.5k damage in 3 seconds. I don't know about you, but I run about 10.6k HP raid buffed, and there's frequent damage from Felmyst's Noxious Fumes aura constantly eating away at that total. If you hesitate, you die. Keep an eye on your distance from others, and know your exit strategy should someone near you get hit with Encapsulate.
After a set period of time, Felmyst will take to the air. The first part of the air phase involves dodging Felmyst's green trail of doom. She uses a targeted breath attack that is easily out-run, but can catch people by surprise. The breath leaves a green trail that spawns skeletons, and if anyone crosses the trail more skeletons spawn. It needs to be said: don't stand in the green stuff. Move away from it. And if the breath attack targets you, kite it. Don't just stand there, you'll die.
After she does the green spew thing twice, she'll decide to carpet bomb entire sections of the area with green gas. Anyone caught in the gas will get irreversibly mind controlled and thus must die. Again, don't stand in the bad things. Keep an eye on Felmyst, she always bombs either the northern half or southern half of the map, and will tip which way she's going seconds before her attack. Run the other way, as fast as your greaves allow.
As you can see, many things can threaten your health in this fight. Give priority to your life-saving abilities. Watch for clothies in danger of becoming skeleton snacks, and BoP them. Lay on Hands a tank in trouble. Divine Shield to save yourself, and only use Avenging Wrath if you are assured safety (Divine Shield on cooldown, Encapsulate just happened, still 20s or more left on the ground phase). Also, since there is so many "move or die" situations in this fight, Pursuit of Justice can be your best friend. At the very least, you absolutely need Cat's Swiftness or Boar's Speed on your boots if you've invested your talents elsewhere.
Besides all that... ummm... pew pew and such?
Special note: Several of the tips in this guide are obsolete because of changes brought in the Echoes of Doom patch, specifically saving Avenging Wrath. My original thoughts on the fight are preserved here for posterity, since this is how I learned it. For your own purposes, you may use Avenging Wrath at any point in the ground phase when you think you'll be able to stand still for the entire duration and pound away at Felmyst's backside.