Karazhan

The Burning Crusade

Awesome 10 man instance located in Deadwind Pass which had an entry attunement quest chain requiring you to fool around outside then go to Old Dalaran in Alterac Mountains and talk to some dude to get some shitty arcane resist trinket as well as doing a few heroics such as Steamvaults, Arcatraz and Black Morass.
You could portal there via Atiesh, Greatstaff of the Guardian.

This instance was the first entry raid for the first tier in TBC. The instance contained 12 bosses with a wide range of mechanics and difficulties. It was tuned very very well for the most part depending on what gear lvl you went into. One of the bosses required a long winding quest chain requiring you to repeatedly clear Karazhan and do a ton of other heroics and travelling all over Outlands to summon a dragon called Nightbane. Arguably considered the last boss of the instance alongside Prince Malchezaar. This instance dropped the first two pieces of Tier 4, the first from Curator which was the Gloves. And second from the last boss Prince Malchezaar which was a Helm token. The Pants and Shoulders for Tier 4 dropped from Gruul's Lair, and the Chest from Magtheridon.

Mad drama ???? <add to me if your not lazy>

Legion

Later on during Legion this raid was reintroduced as a 5man Mythic dungeon which later became available to run in Mythic+ and LFG finder queues. It had the same feel for attunement and aesthetic as the original with a few route twists. Some boss fights felt very much the same and contained some fairly difficult mechanics just like the original. To summon Nightbane you had to do a bunch of interesting things to force him to spawn. These things consisted of killing the Opera event within 8 minutes of opening the main entrance door and channeling the first Soul Fragment crystal. From then on you had 5 minutes to click on the remaining 4 crystals which would extend the duration of the buff. Once you have a member with 4 of those buffs, channeling on the last one will transform the buff into a new one. You quickly jump down back to Nightbane's platform and you can then start the fairly difficult encounter. Nightbane was fairly unique because it dropped a chest piece for each armor specialization which would empower other trinkets you could get throughout the instance. Nightbane also dropped a ton of different relics so it was fairly beneficial to go run it each week.

Another great addition to Return to Karazhan was a grey item dropped from Opera trash called Negative Review. These were constantly mailed around to each other whenever somebody got one to drop.

Return to Karazhan: Upper Mythic+ consisting of; Curator, Echo of Medhiv, Mana Devourer, and Vizadum. Was notorious for only taking 16 minutes to clear! Special Shout outs to the run just after release (on base mythic) where Mow and Dippy had to leave the instance without completing it Because Moopheus and Jcal couldn't do the last boss mechanics.