Nightbane made a return along with Karazhan in patch 7.1. He is an optional timed boss that awards the players with the mount Smoldering Ember Wyrm, but getting him to spawn within the timer can be pretty challenging, especially for players that are not very well geared.
Before going into the instance, get ready by eating food and buffing up, because the first timers starts once the first player in the group goes into the instance. You can see that the timer starts by seeing the emote "The strange chill of a dark presence winds through the air". For the first part, you need to reach and kill the Opera encounter within 8 minutes, and click on the Soul Fragment that is located in the audience. If you are too slow, an emote saying "The air grows slightly warmer", which means you failed, and need to wait for the next reset to try again.
If you manage to click on the Soul Fragment within 8 minutes, the group gets a buff called Medivh's Echo. This has a 6 minute duration, and you need to find a new Soul Fragment in order to extent the time of the buff. Each new Soul Fragment adds another 5 minutes to the buff.
The location of the Soul Fragments are as following:
1) In the audience in the Opera event.
2) Before Maiden of Virtue. (You can click it without killing the boss).
3) Right after killing Moroes.
4) In the Spider Room before you take the portal to the stairway.
5) Right after you kill Curator.
Once you have clicked all five, the group get's a new buff called Medivh's Presence. This means that Medivh will be waiting in the Nightbane room, but hurry, you only have 5 minutes before the buff disappears. Speak to Medivh to give everyone in the group the "One Night in Karazhan" achievement, and he will begin to summon Nightbane. This takes a little while with some event going on before Nightbane appears, but don't worry, you don't have any more timers pushing you!
Once Nightbane has spawned, all timers are gone, so don't worry if you wipe or spend too long to down him. Once defeated, he will drop the Smoldering Ember Wyrm to all party members that does not already have the mount, as well as having a chance to drop a trinket, a chestpiece for all users, or Artifact Weapon Relics.
Good luck! Most people will probably need a few tries to be able to beat the timer, because this one is difficult unless your insanely good geared.