This could be the rose-colored glasses of nostalgia speaking, but looking back, I really do believe that The Burning Crusade had some of the most interesting, fun, and infuriating boss mechanics of any expansion so far. Mage and/or warlock tanks? Check. Necessary item usage? Check. Guaranteed aggro resets? Check!
And Kael'thas had all of those.
Kael'thas was a five-phase fight, and you didn't even touch him until phase four! In phase one, his four advisors came at you one by one, at half health, and you killed them. This phase was notoriously unfriendly to melee DPS, and required a warlock tank for Capernian. Raiders new to the fight also quickly discovered that where you kill the advisors was critical to the success of the entire encounter because you fought them twice. But I'm getting ahead of myself; we're not there yet.
In phase two, Kael'thas summons seven legendary weapons: a sword, a dagger, a bow, an axe, a shield, a mace, and a staff, that attack the raid. Phase two is timed and lasts two minutes. Again, positioning during this phase was extremely important, because some of the weapons (notably the axe and the bow) did attacks that could be one-shots. As the weapons died, they became lootable. Equipping the weapons and using their special abilities was essential to the successful completion of the subsequent phases.
Once phase two's two minutes were up, phase three popped. Kael'thas simultaneously resurrects all four of the advisors you killed in phase one, all at full health this time. You are still on a timer: you have three minutes to complete this phase before phase four begins. This resurrection is the reason positioning is so important in phase one. Because your raid will now be fighting all advisors at once, where they are in the room affects how efficiently and cleanly they die again. The good news? You now have the legendary weapons, which do things like reduce incoming damage (the mace), boost attack speed (the sword), and absorb damage (the shield). Thanks to the aid the weapons provide, phase three is doable.
Phase four was the kicker. In phase four, you at last face Kael'thas himself. He summons pheonix adds (which in turn can spawn more phoenix adds), casts Mind Control on random members of the raid (which must be broken using the legendary dagger), and leaves very nasty Flame Strikes all over. But the mechanic that really made me boggle when I was preparing for tier 5 was the Shock Barrier + Pyroblast combo.
Every sixty seconds Kael'thas casts a Shock Barrier around himself that absorbs all damage done and lasts for ten seconds. After he puts up the Shock Barrier, he casts three successive Pyroblasts, which hit for ~20,000 damage each. In TBC that was a one-shot. The Pyroblasts had a four second cast time, and it was impossible to have the barrier down in time to interrupt the first one, so the tank had to use the legendary shield's absorption ability to soak it. The second Pyroblast would hit at eight seconds, two seconds before the Shock Barrier would wear off. Thus the raid's DPS had ~7 seconds to kill the Shock Barrier, giving a one-second window in which to interrupt the Pyroblast and save the tank.
If the raid couldn't get the barrier down in time to interrupt, it was just possible for the tank to survive it using things like Shield Wall, Nightmare Seed, or other emergency damage-reduction/HP-boosting items.
Kael'thas was a guild-breaker boss par excellence. It was a long, brutal fight, requiring excellent coordination, DPS output, as well as patience if, say, a tank got the Remote Toy debuff during phase one. The worst part was that Kael'thas was a content gate. For most of TBC, if you couldn't kill both Vashj and Kael, you physically could not enter the tier 6 raids. Completing the attunement quest for Battle of Mount Hyjal required items off of both Vashj and Kael, and the attunement for Black Temple required killing the first boss of Battle for Mount Hyjal. Vashj was no cakewalk herself (Tainted Cores, anyone?), but raid statistics from the time showed that more guilds could take her down than Kael'thas, so it was Kael that became the biggest hurdle to tier 6 content.
Players who managed to complete both attunements were rewarded with the Hand of A'dal title, and when you saw someone with it, you knew they were a skilled raider. Many, many guilds were dashed to pieces on the crucible of acquiring those Vials of Eternity. For the very unlucky, failing to down Vashj and Kael'thas - raids present at TBC's release - meant that you wouldn't get to see any new raid content other than Zul'Aman until the last patch of the expansion, 2.4, which removed the attunement requirement from both Hyjal and Black Temple.
There were a lot of memorable fights from Burning Crusade. Other contenders for this article were Shade of Aran, Lady Vashj, Leotheras the Blind, Illidari Council, Illidan, Mu'ru/Entropius, and Eredar Twins. Ultimately I had to pick Kael'thas because I've always felt that his raid mechanics were both particularly clever and particularly devious. The requirement to equip and use the weapons you defeat in phase two is, in my mind, an ingenious bit of encounter design, though I can't say that I'm sad it hasn't really been replicated since. Getting everything to go just right in the Kael'thas fight was like mastering a complex piece for the piano - it took ages to coordinate your hands and hit the right notes, but once you did it, the result was lovely. And it really was a question of skill. Once you'd mastered the fight, it was pretty much entirely reliable, with no big RNG bombs to mess it up. In fact, it was so predictable that I heard more than one advanced TBC raider say they got bored with it after a time, and preferred Vashj, because her more random elements kept the fight fresher.
I'm not sad that there haven't been any other fights like Kael'thas - one was enough. I am, however, glad it was there, because it's great to remember and write about.