In Patch 7.1, Blizzard returned to a fan-favourite instance from The Burning Crusade: Karazhan. The new Karazhan instance is a call-back to the old one, but is designed for a 5-person group, rather than a raid.
Return to Karazhan is also part of an interesting new trend in Legion: a move away from transient content, and towards more extended content. Return to Karazhan are is the third Mythic-only dungeon, and then there are also Mythic+ dungeons. Personally, I think it's a good trend, shifting the balance back. Regular heroic versions of these dungeons are coming in a later patch.
Return to Karazhan is also a much longer dungeon than we've seen in a very long time, maybe not since Blackrock Depths in Vanilla. The first time I went into Kara with a guild group, we only managed to kill 4 bosses in 4 hours. As one guildie put it, "[Kara] isn't a 5-man dungeon, it's a 5-man raid!"
The next week we did better, of course, and managed to down all 8 bosses in 4 hours. But it's still a very long instance, and would be a good candidate for a smaller guild to do over a couple of nights.
The bosses are all very well-done. They're almost all call-backs to the original bosses and share similar themes. Old-school players will recognize most of the fights, but the mechanics are all very well designed to work with a 5-man group instead of a raid.
The first half of the instance is fairly normal, but the second half has this crazy Alice-in-Wonderland feel to it. For example, for one part, you're shrunk down to a tiny size and have to deal with trash like a single normal rat (who hits like a truck, by the way) or a single spider. The boss of this area is a single Mana Devourer, which is a weak trash mob in the rest of the instance which you usually kill multiples at a time. That whole area just makes me smile when remembering it.
About the only negative thing I can say about Return to Karazhan is that the Chess event is not very good. It's kind of boring, takes up a lot of time, and it isn't even a boss with loot. It also comes at the very end of the instance where you just want to get to the final boss. It feels like something that should have been cut, but was left in because everyone would have complained if Return to Karazhan didn't have a Chess event like old Karazhan.
But all in all, Return to Karazhan is an excellent instance.