Here's the Terrace highlighted on your map of the Isle of Quel'Danas.
Ooh, glowy...go on, walk in. You know you want to.
Alright, so you've formed your group and stepped inside the instance. This nice frieze of Kael'Thas Sunstrider is the first thing you'll see. Go left or right around it to find your first pull.
It's only a 2-pull, with two Sunblade Mage Guards; shouldn't be any problem. There is also a pat at this point, so watch out for that. The Mage Guards have a stun that can jump from the target to other nearby characters, so stay back if you're ranged.
In addition, Mage Guards are what make the sparkly pink bubbles you see throughout the instance. Anything inside the field will receive 75% reduced effect from both healing and damaging magic spells cast upon them, so it is paramount to make sure both the mobs being DPSed and the tank stay outside the bubble. In theory it might help to keep your healer and ranged DPSers inside the bubble, to reduce the amount of AoE damage they take (there's a fair bit of AoE in MrT), but on the other hand, they'd need to remember to move out of the bubble when they wanted heals. Probably best to just avoid them altogether, like almost any other highlighted area in this game (Flame Wreath, anyone?).
After the first pull is a good time to take out the pat. It's another 2-pull composed of a Sunblade Physician and a Sunblade Magister. Now, contrary to what you would have been led to expect by both WoW and common sense, the Physician's don't heal (much). They do, however, have a poison that hits for 1.2-1.4k. And they use Prayer of Mending; you can dispel that if you're quick about it. The Magisters use Arcane Explosion and Frost Bolt, and gain haste every time they cast a spell. Mages can steal this haste buff if they so desire. Magisters are immune to seduce, but can be sheeped.
Once you've dispatched the pat, you come to the first pulls that might put up a bit of a challenge. There are three four-pulls, one on either side and one a bit past that. They're composed of Mage Guards, Physicians, Blood Knights, Magisters, and Warlocks (with imps). The imps are easy enough to kill. The Warlocks cast Incinerate as well as a fairly nasty Immolate; be sure to dispel that as soon as possible. They have a stealable +250 spell damage Fel Armor buff. The Blood Knights have a pretty sizable Holy Light; interrupt that if at all possible, and try to kill them early.
The Blood Knights are paladins, of course. They heal and have Seal of Wrath, which adds to their spell and melee damage; you can dispel it off them or let a mage steal it (it helps quite a bit, and applies to every tick of Arcane Missiles for a total bonus of 8000 damage over the course of the spell). Your CCers should be doing all they can by this point. Kill priorities are probably Blood Knight first, then Physician, then whatever.
After that last four-pull is the first boss's room. On either side are six level 68 elites clustered around crystals, plus two in the middle. They're not that hard, but they do hit cloth pretty good, so AoEing is "not recommended" (as WoWWiki puts it). Just CC what you can and pick them off one-by-one.
Selin Fireheart is, as everyone points out, a lot like Kalithresh, the last boss of the Steamvaults. He starts out with an empty mana bar, and occasionally goes around to the pointy red crystals distributed around his room and siphons mana off them. When he does this, it is paramount to kill the crystals immediately. Any mana he does manage to get off them will be released as an AoE which hits everyone in the room. Make sure your entire group is in the room before engaging Selin, because the room seals when the fight starts. He also does a drain life, which targets a random character and should be interrupted if possible. Other than the crystals, though, it's a very straightforward fight, and should not pose much challenge as long as your DPSers are on their toes.
Selin drops Sun-forged Cleaver, Gauntlets of Divine Blessings, Leggings of the Betrayed, Bracers of the Forest Stalker, Bindings of Raging Fire, and Cloak of Swift Mending (which is white, and longer than your average cape or cloak).
After defeating him, a door opens and you can proceed to the rest of the instance. The turn-in for the quest Magister's Terrace is shortly down this hallway; click the dying elf and receive the followup, which you will turn in after the next boss.
The next trash you face will be a very large amount of these tiny, non-elite Brightscale Wyrms. It's an AoE-fest and everyone's invited! Killing the wyrms gives you a stacking but brief +damage buff, 1% for each wyrm killed, which is fun (stacks up to 25).
After blowing through the Wyrms, you'll get to the second boss, Vexallus. The immediate comparison here is to the Curator from Karazhan: he periodically spawns an add that needs to be DPSed down lest it chain-lightning everyone to death. The adds don't have much health; however, the person who kills them gets a de/buff, which ticks for 300 damage per second but increases the damage they do by 50%. It might be best for the DPS classes to rotate who kills the adds, so that nobody's dropping in health too fast. When Vexallus gets to 10% health, he Overloads and starts AoEing the heck out of your party. At this point you need to just burn him down as fast as possible; save your cooldowns for this part of the fight.
Vexallus's drops are Band of Determination, Band of Arcane Alacrity, Bracers of Divine Infusion, Latro's Dancing Blade, Cloak of the Coming Night, and Boots of Resuscitation. Congrats, you're halfway through! But that was the easy half.
As a bit of an intermission, it's time for a cinematic! Click the scrying orb which you'll see on the balcony past Vexallus, and you'll be treated a neat preview of the Sunwell Plateau and the guys trying to summon Kil'jaeden.
The blue dragon Kalecgos then soars in and morphs into a human, at which point you can turn in your quest and get the final follow-up to kill Kael'thas himself.
Now that we're all refreshed, it's time to move towards the first courtyard of hell-pulls. These pulls tend to be five mobs, and all the advice I can offer you here is CC what you can, and have an excellent tank. I died more on these trash pulls (and the next boss, which is similar in many was) than on the rest of the instance put together. This is also where line-of-sight pulls become crucial: aggro the mobs, either by some ranged means or by running up to them, and then have everybody move around behind a wall so the mobs all come running to you. Otherwise you're virtually guaranteed to get adds.In terms of group composition, the mobs here are some of the elves from earlier in the instance, as well as some new mobs. Ethereum Smugglers do a lot of AoE, so are they're a high priority to kill or CC, and be sure to spread out when facing these. But the number one mob to take out or incapacitate, in my opinion, is the Sister of Torment, which is a succubus, complete with a Seduce-type ability. Kill or banish ASAP.You don't necessarily have to kill all the pulls in the first courtyard, since you can just pick a side (right or left) and clear that way, as well as the one pat.
After the courtyard is a Sunblade Sentinel, which does a chain lightning, but that's nothing compared to the pulls you just faced and the ones that are about to come. Just kill it and move on.
Welcome to the second courtyard! Here you can see the third boss(es), Princess Delrissa and her posse. But before you can face her you have to clear some more rather difficult pulls, just like in the previous room. Good luck. In addition to the 5-pulls, there are small groups in the corners of the room; you can take these out too if you want, but you don't have to, unless you see Ellrys Duskhallow or Warlord Salaris up in the center. They cast fear, so clear the entire front of the room if they are present. In fact, you may want to go around and clear the back of the room as well, but be careful to stick to the wall so as not to aggro the boss group (this happened to me).
Once you've cleared as much as you need to, it's time for another boss fight. Continuing the analogies to other bosses, this fight is partly similar to Moroes from Karazhan, in that there's a main boss plus four sub-bosses randomly chosen from a list (of eight, in this case). However, the strategy differs considerably, and the most valid comparison that I've seen made is to a 5-man Arena fight. As such, it might help to have your DPS put on Arena gear, if they have any, for more survivability. For notes on the abilities of the adds, consult WoWWiki. However, in general, you just want to CC as many of the adds as possible and kill them one at a time. Make sure your group members have health potions and health stones if possible, because everybody will probably be taking some damage. And if you're the healer, be ready to heal anyone that might need it.The adds are immune to taunt and in general do not seem to have much of an aggro table, so expect them to basically attack whoever they feel like. It's very unlikely that your tank will be able to hold them all. The Priestess herself is not terribly difficult, but she does heal, so it might be best to kill her first. Warlord Salaris fears and apparently does heavy damage, so he's another good one to focus on.Another strategy that may work is to bunch up the adds and AoE them, but I expect this is probably gear- and class-dependent. All in all this is a very chaotic fight and I have yet to see any clear, good strategy emerge. Please let me know if you have one.
Delrissa's drops: Duskhallow Mantle, Gauntlets of the Tranquil Waves, Nightstrike, Bracers of Slaughter, Cloak of the Betrayed, Battle-mace of the High Priestess.If you haven't cleared the back of the room yet, you'll have to clear at least some of it to move forwards. More big pulls, I'm afraid; and if you're hungry for rep there are some small pulls in the corners again.
Almost to the big boss, but not quite. There's one more trash pull left, and it's a strong contender for worst trash pull in a normal 5-man dungeon. It has six mobs (plus an imp): Warlock plus imp, Magister, Blood Knight, Coilskar Witch, Ethereum Smuggler, Sister of Torment. This pull is easily as hard as the first two bosses, in my opinion (which is not saying all that much, as they are very easy bosses, but I did wipe on this pull). And as a bonus, if you don't kill every mob, they all reset, so partial kills will do you no good. CC what you can, interrupt what you can, kill them all, and stay alive. Be careful of the Smuggler's AoE, the Sister's seduce, and the Blood Knight's heals. It might be beneficial to drag these mobs forwards into Kael's room to get more room to fight and avoid the AoEs. Be careful not to aggro Kael, but there is a fair amount of room here.
And finally, we have Kael'Thas himself (in a weakened state). There are two phases to this fight, and two major abilities to watch out for: Summon Phoenix, Flamestrike, and Gravity Lapse. He will also Fireball random party members throughout the fight for 2-3k. When the Phoenix spawns, kite it around (it does AoE damage), and it will eventually die. It will now turn into an egg. You have to kill the egg at this point, or else the Phoenix will come right back. Not too difficult, just something to watch out for. The Flamestrike is an easily visible fiery area. Stay out of it. When Kael'Thas gets down to 50% health, he will cast Gravity Lapse, nullifying the gravity in the room and making it act like water. The number-one thing to know here: stay away from the sparkly bubbles. If you touch one, you lose 2,000 health. There's three of them, and they move around the room. Everyone needs to be alert, or you'll be losing group members in no time. It is rumored that if you touch the ground you get stunned (which may cause a sphere to run into you), but I can't verify this firsthand. Each group member will also be getting hit with a small but constant stream of damage from Kael's Magic Missiles. This is where group heals really shine; finally, I'm glad to be playing a Holy priest. It is a hard bit to heal through, what with everyone taking damage, some people blundering into the spheres, and having to move around yourself to not get hit. So this is a good place for hybrid healers to throw some heals in. An interesting strategy from a Wowhead comment: "The best tactic I found for this stage was to have your entire group "swim" to the front of the room where the door is, cast some DoT's/HoT's/Direct Damage spells and when the orbs get close everyone swim to the front and left side of the room, repeat damage. Then move to the back, repeating damage, then to the other side of the room. Always staying ahead of the orbs. By now the Gravity Lapse is over and you have about 10 seconds to DPS Kael hard before he casts the Gravity Lapse again." I'll have to try this at some point. Note that there is no tanking Kael'thas at this part of the fight, so feel free to DPS as hard as you can, and tanks, if you want to strap on a DPS weapon for now, go for it. For many players it will be all they can do to stay alive while adding a bit of DPS, but all you can do here will help (save your cooldowns until after the Lapse though). The primary thing is to stay alive though, since dead players do no DPS later. Eventually Gravity Lapse will end, and Kael'Thas will be stunned for a short time, during which he does nothing and takes 50% increased damage. Burn your cooldowns and go all-out on DPS at this point, because he does another Lapse at 25% and one at 10%, but if you can get him down below 25% during this first stunned period, he'll only do the 10% one and you've saved a Gravity Lapse. But you'll be facing at least one more Lapse, so be prepared for that. Same as before. Note that he doesn't seem to have a ton of health; one Warlock I know (in T5 gear, admittedly) managed to solo Kael down from 38%.
Woo! Kael dead, and you're done! For dealing with those darn Gravity Lapses, you are given one rare item and one epic. Also, everyone be sure to loot his head for Hard to Kill. There is an Orb of Translocation at the back of the room which conveniently zaps you back to the Staging Area, so you can turn in your quest for an epic gem and access to the Terrace on Heroic. After all, don't you want to try that again, but harder this time?