Everyone who played Vanilla WoW remembers Blackwing Lair. Maybe you didn't actually go in because you weren't lucky enough to wrangle 39 other people or didn't have the time: but you probably remember when it came out and the insane ripple effect it had on its userbase.
Although it was preceded by Onyxia's Lair and Molten Core, Blackwing Lair had a different, more intimidating air around it when it was added in patch 1.6.0 back in July of 2005. But this is 2020, and it looks like Nefarian is relevant all over again. Packed with eight bosses and the obvious level 60 requirement, Blackwing Lair is now out in World of Warcraft Classic. In case you forgot where it is and want to explore the entrance, you can find it a the tip of Blackrock Spire.
As a reminder, attunements are still a thing in WoW Classic. Simply put: you have to earn the right to enter the instance, invoking the ire of casual players everywhere (Blizzard continued this practice into the next expansion but eventually eliminated it). You have to best the Scarshield Quartermaster, then grab a new questline from an item looted from his corpse. You can find an official walkthrough here.
If you're diving back in or diving in for the first time, good luck! Patch 1.6.0 is arguably one of the highest points of the game outside of 1.9.0 when the Temple of Ahn'Qiraj (AQ40) was added.