Upon opening the door after zoning into Karazhan, the following emote happens:"The strange chill of a dark presence winds through the air." This starts the timer.
You have a STRICT timing to make it to Opera, kill Opera and click the first Soul Fragment. You have 8 minutes from the moment you zone in, to click the first crystal.
From there on each Soul Fragment you click add a bit of time to your timer. But its very very tight.
If you fail, you will get an emote "The air grows slightly warmer".
After the first crystal you will gain a buff, Medivh's Echo, with a 6min duration. This is now your timer. As you click crystals, it will add a stacking buff and 5 minutes to the time you had left on your buff.
The Soul Fragments are located: 1 in the audience from Opera, 1 before Maiden (you don’t need to kill her), 1 after you kill Moroes, 1 in the Spider Room (then you click the portal to the stairway), 1 after you kill Curator.
If you click all the Soul Fragment within the allotted time, after Curator, you run back to Nightbane's room and Medivh is there and he summons Nightbane.
When you speak to Medivh you get the One Night in Karazhan achievement.
Once all crystals are clicked in the allotted time, your timer buff will change to Medivh's Presence and you will have 5 minutes to go speak with Medivh and summon Nightbane. The original Roleplay upon summoning Nightbane will play.
Once you speak to Medivh and Nightbane is summoned, you are able to wipe without worry. He will still be there and shout at you as he continuously slays you.