Demons die when a lore nerd goes to war.
QQs will sound and drown the fun
When a lore nerd goes to war.
Each PUG dies and ninja lies
Tank will quit and the the queue will rise
When a lore nerd goes to war.
Demons die, but count the cost,
The valor's won, but the cheevy's lost. -- Bellajtok
Well of Eternity is chronologically the second of the three dungeons you'll encounter in patch 4.3, but it was also the last of the three to become available on the PTR. There were also a few bugs with the second encounter when it went live, so you probably won't find as many useful videos or things online about it. However, it's a pretty cool dungeon, with quite possibly the funniest 5-man achievement in the game.
Initial trash in the instance isn't really anything to worry about. Illidan will escort you where you need to go -- and that enormous line of demons you'll see marching around the courtyard? He'll stealth you past most of them. The Shadowcloak buff he'll give you to get past them even gives you a big crit buff. The only trash packs you'll need to worry about are the ones around the three portals you have to destroy. They're not much of a concern, though the DPS and healer should stay out from in front of them due to a nasty cleave. The crit buff will give you some stupendous numbers here.
The only other trash you'll have to take care of are two felguard mobs (fought individually) with a stacking AoE damage spell that, like Peroth'arn's Corrupting Touch, is basically an enrage timer.
This is an interesting encounter with a mechanic that will probably drive you crazy in a PUG. Actually, the same is true of all the boss fights in Well of Eternity, so get used to it.
Peroth'arn in 5 seconds or less Stay out of green stuff on the ground, and don't let the eyes get close to you when you're stealthed.
Corrupting Touch Peroth'arn will periodically apply a debuff to the tank increasing damage taken by 10% for each stack. In essence, it's an enrage timer, although not a particularly punishing one.
Fel Flames Green fire that will spawn under players and does a surprising amount of damage. You are advised not to stand in it.
Fel Decay This is primarily the healer's concern. The debuff applied to a random player deals an initial hit and then a 10-second DoT. Half of whatever you decide to heal will also be dealt to you as damage. If the player was at full or close to full health beforehand, you may want to leave him alone until the debuff wears off.
Drain Essence At 70% and before Peroth'arn starts the stealth-and-eyes phase (see below), he'll stun the full group and deal damage to everyone. It's unavoidable, so if you're the healer, try to get people topped off before it happens.
Stealth and eyes After Drain Essence, Peroth'arn will disappear and players will reenter Shadowcloak (Illidan's stealth spell). After a few seconds, green eyes (exactly like the warlock's Eye of Kilrogg, but bigger) will spawn and start looking for players. They're not very fast, although they're a lot more likely to detect the tank and any melee DPS who were obviously closer to Peroth'arn when he disappeared. If they do "see" you, Peroth'arn will unstealth and start attacking the player in question. Needless to say, this is bad.
Endless Frenzy When his health hits 20%, Peroth'arn starts hitting faster.
Tank This is pretty straightforward to tank. You may want to blow some cooldowns toward the end to help with healing when Peroth'arn frenzies, but otherwise your primary responsibility is staying out of bad stuff and running away from the center of the circle once you're stealthed. The eyes are most likely to see you, as you don't have much choice over being close to Peroth'arn when he disappears. As soon as he does, take off.
DPS You have to kill this guy before Corrupting Touch stacks too high, but that shouldn't be an issue if you meet the ilevel requirement for this place. If you're melee, you may want to preemptively run from Peroth'arn slightly before 70% to reduce the chance of being detected by an eye during the stealth phase.
Healer Peroth'arn doesn't really hit too hard (although his damage will increase over the course of the fight with Corrupting Touch and Endless Frenzy). How you deal with Fel Decay is up to you. I found I could ignore it entirely on players who were full or close to full when it hit, but it was necessary to heal somewhat on anyone who kept getting caught unawares by Fel Flames.
Following the spiral up from Nozdormu will lead you to the Azshara encounter. The only trash of concern here is the set of eyes that will look like the ones you just encountered on Peroth'arn with a Chain Lightningesque ability, but they shouldn't be a major problem. If they are, you may have some issues getting past Azshara.
For all you tier 5 raiders out there, the Azshara fight bears a resemblance to the Tempest Keep version of Kael'thas. Hands up, all those of you who relish the prospect of dealing with those mechanics in a PUG.
Queen Azshara in 5 seconds or less Kill the puppet strings on Mind-Controlled players as quickly as possible and avoid bad stuff on the ground. Interrupt Total Obedience from Azshara.
Shroud of Luminosity You won't actually be able to damage or even fight Azshara one bit. Your targets here are her Enchanted Magi adds.
Hand of the Queen This is the first of two Mind Control spells that Azshara can cast, but this one's only on a single target. Periodically you'll see a giant hand controlling Puppet Strings attached to a player, who will be Mind-Controlled and almost certainly beating the crap out of your party's healer. Neither the Hand nor String appear as a debuff, but it's pretty obvious who's affected by it with the big white hand hovering over the player's name plate. Kill the Strings as quickly as possible.
Total Obedience This is the AoE Mind Control that has to be interrupted. Failure to do so means the fight is over and Azshara will kill you each one by one. She won't cast this often, but perhaps once or at most twice per fight assuming your DPS has the gear for the new 5-mans. The ability will also be announced and it's an 8-second cast, so you've got plenty of time to get to it.
Enchanted Magi These are the adds you'll be fighting, and there are two each of the arcane, fire, and frost magi. They become active in sets of two (never the same school of magic at the same time from what I've seen, although that might just have been random) and must be tanked. You can interrupt the fire mage's Fireball, the arcane mage's Arcane Shock, and the frost mage's Ice Fling. Apart from that, the two AoE spells to avoid are the arcane mage's Arcane Bomb and the frost mage's Coldflame. Arcane Bomb will announce its presence ahead of time with a ground effect and a bomb descending from the sky; Coldflame is pretty obvious with a big line of white extending on the ground. Keep in mind that become active on a set schedule (much like the Kael'thas encounter), so if your DPS is slow, you'll have more of them to deal with.
Tanks This can be a pain to tank. Sometimes the Enchanted Magi adds will take off for your party's healer (this seems to be most likely with the arcane mages). Sometimes they'll start casting immediately after becoming active. However, they seem to become active in the same order, with the central two attacking, then the ones on either side of them, and then finally the magi at either end of the line. Whether you or a DPSer interrupts Azshara's Total Obedience is up to you, but it's generally easier for a DPSer to do it.
DPS Your primary responsibility is to get Mind-Controlled players back to normal as fast as possible. The most consistent issue I saw on this with PUGs was tunnel vision on add damage while Mind-Controlled players stomped the party's healer. Arrange in advance for one person to interrupt Total Obedience. Of course, if said person is Mind-Controlled at the time, someone else will have to take care of it.
Healer Mind-Controlled players obey normal threat rules, and they'll make a beeline for you, so beware. There's a lot of random damage on this encounter, and I would actually consider it a bit of a gear check for healers.
Captain Varo'then and Mannoroth
After your tiff with Azshara, you'll hop on bronze drakes for a small cutscene, and then fly to the Well of Eternity for the final fight. The Captain Varo'then/Mannoroth fight is a multi-stage encounter with no trash beforehand but plenty of trash during the actual fight itself.
It probably makes more sense to describe the fight in terms of its phases than by specific abilities. It's more than a little chaotic and I'm positive I've missed some of the finer details as I've only ever healed it, but it's pretty fun.
Captain Varo'then and Mannoroth in less than 5 seconds Stay out of bad stuff, focus on Mannoroth once Varo'then's dead, and keep adds off your healer and Tyrande.
Phase 1 Before the fight actually starts, have everyone except the tank wait in Tyrande's moon circle (it gives a buff). The tank can go run and collect demon adds. Once these are AoEed down in the moon circle, you'll fight Varo'then while Illidan runs off to engage Mannoroth. Varo'then is largely unremarkable except for an ability called Magistrike, which adds fire damage to his next hit (and also seems to hit nearby players).
Phase 2 Once Varo'then's dead, his sword drops to the ground. The next phase of the encounter starts when you toss it at Mannoroth. He has a tremendous amount of health, but due to the Embedded Blade debuff, attacks to him have a chance to cause an additional 200,000 arcane damage to him. The only ability I noticed here that really caused any problems was Fel Firestorm, which leaves pulsing green fire on the ground and should be avoided. At one point (or possibly earlier, it was hard to tell) demons started pouring out of a portal and (unsurprisingly) head for your healer.
Phase 3 When Mannoroth hits 20%, Illidan will buff you with an AoE fire damage aura and transform you into something resembling the warlock's Metamorphosis.
Tank You'll mostly be concerned with adds, Varo'then, and then more adds, in that order. Mannoroth remains taunted to Illidan, so you don't need to worry about tanking him at all. Keep adds off your healer and Tyrande. As Shandris in the comments observed, Illidan has an AOE buff decreasing fire damage taken, and he's a good person to stand on or near while Mannoroth casts Fel Firestorm.
DPS Don't worry about the adds once Mannoroth is vulnerable (i.e., once you've tossed Varo'then's sword at him) -- just focus on getting him down. Stay close to Illidan for the same reason given in the tank section above.