Last week, my guild group that has been running Karazhan semi-regularly unlocked and beat the "secret" boss, Nightbane.
The unlocking mechanism is basically a speed run. There are 6 or so crystals scattered around Karazhan, and you have about 6 minutes to reach each crystal in turn. It's mostly trash, as you only have to kill 3 bosses (Opera, Moroes, and Curator). We even use invisibility potions at one point to skip a couple packs of particularly nasty trash.
It was pretty fun, though I am not super-thrilled with Blizzard making speed runs the only challenge in lower content. I haven't done a single Mythic+ yet, for example. But I don't know. So far, speed runs have proven to be the only enduring challenge in low level content. And perhaps having speed runs filters the "go-go-go" crowd out of regular dungeons.
Nightbane itself is a pretty straightforward fight. It's very similar to the old Nightbane with a dragon, skeletons, and charred earth zones on the ground.
The one really interesting mechanic is Ignite Soul. Ignite Soul is a debuff which targets one player and lasts for 9 seconds. On expiry, damage equal to the target's current health is dealt to the other players in the group. So the player with Ignite Soul has to stand in the charred earth and get her health down to 25% or so when the buff expires. So the healer has to watch them, avoid healing them (but don't let them die), and heal everyone else up high enough to take the coming hit.
It's a neat mechanic. The fight overall is quite decent. It even has Nightbane fearing everyone, just like the old fight. Though this time, I don't think anyone can break it early consistently. Heh, that brings up memories. I think the fear was what I complained about in the original fight.
Nightbane also drops a mount. The loot mechanism is interesting. If no one in the group has already killed Nightbane this week, it's a 100% drop for one of the players who don't have the mount. Otherwise the chance of the mount dropping decreases by 20% for each person who has already done Kara. So it's really aimed at people who run Kara once a week in a steady group, but allows a group of 4 who already have the mount to guarantee the mount for the 5th person.
All in all, Nightbane was an good fight. It's a bit unfortunate that it's locked behind a speed run, but the run is pretty doable with a decent group. It's actually a decent challenge for a stable 5-man group that doesn't raid.