Prince Tortheldrin
Prince Tortheldrin is the prince of the Shen'Dralar night elf faction, who has stood guard over the demon Immolthar in Dire Maul for the last several thousand years. He is friendly by default, you can speak with him in the Library section. He turns hostile once you kill Immolthar and you can fight him.
The reason he is mentioned here is because one of his acolytes in the Library mentions that the prince knows the location of the sword. Screenshot This would seem to indicate that some object or action in Blackwing Lair could trigger the prince to tell his story, and then theoreticly start the quest to attain the sword.
Unfortunatly nothing so far has made the prince change his tune. Players have tried talking to the prince with the Head of Nefarian, but with no results. They have also tried it with the Head of the Broodlord Lashlayer, with no effect. Other attempts have been tried in combination with a full Tier 2 armor set, the head of onyxia and Nefarian's, with various levels of faction with the Shen'Dralar.
Some speculate that a rare drop may have yet to drop from Nefarian, but this is unlikely at this point, since the instance has been active for over 6 months now, and run by countless hundreds of guilds. The most likely scenario now is that once a particular quest has been completed elsewhere, or is active during a Blackwing Lair run, a drop will occur that will allow you to speak to the Prince in detail. Once Naxxramus is patched in, we shall see.....