This is the final boss in Karazhan a fiery bone dragon that was created when Medivh attempted to kill this dragon with a powerful spell that burnt the dragon from the inside. The dragon died but its bones were enchanted with Medivh's magic and formed into Nightbane.
In order to summon him you must do a long quest line which includes killing Shade of Aran, Prince Malchezaar, Heroic Shattered Halls, and Heroic Sethekk Halls.
Anyways this video shows two of our attempts on the Dragon, and shows you what to expect in The Burning Crusade as far as boss encounters go. No, we do not kill him because he is quite hard. It is sped up to 1.2x so that its not as boring.
Our best attempt was 50% when he flys again, but I was unable to get it on video :|.