After the destruction of the Well of Eternity many Highborne slipped into lethargy. So it came to pass that Prince Tortheldrin ordered the building of pylons. These pylons should create a force field that could imprison the mighty demonImmol'thar. To feed their hunger for magic the Shen'dralar learned quickly the skill of siphoning the demonic energies of Immol'thar. The energies are used to grant immortality to these Highborne elves.
Centuries later, the powers that imprisoned Immol'thar and controlled the pylons began to wane. So Prince Tortheldrin decided to kill some Highborne to maintain his source of unlimited power.
The only way of killing the prince is to destroy the guardians surrounding the pylons to lower the force field. When this is done, destroy Immol'thar. With the Prince's power greatly reduced, you may attack and end him.
Tortheldrin was implied to have known the location of Ashbringer before Naxxramas was implemented. Furthermore, the Shen'dralar Zealots believed that if someone would defeat Nefarian and tell the tale to Tortheldrin, he would reveal Ashbringer's location, as Tortheldrin had a great interest in Blackwing Lair.
Quotes
When talking to him before angering him
For 10,000 years I have ruled this kingdom. In that time, I have heard many fascinating tales from those that would walk the hallowed grounds of the House of Shen'dralar.
As I am bound to this place, I often exchange stories with visitors. Do you have a story for me or perhaps you would like to hear a story?
After killing Immol'thar
Who dares disrupt the sanctity of Eldre'Thalas? Face me, cowards!
Abilities
Arcane Blast (Blasts an enemy with Arcane magic, inflicting normal damage plus 875 to 1125 and knocking the enemy back.)
Counterspell (Counters an enemy's spell, preventing the enemy from casting that spell again for 15 sec. Generates a high amount of threat.)
Thrash (Gives the caster 2 extra attacks.)
Whirlwind (Attacks nearby enemies in a whirlwind of steel that inflicts normal damage plus a bonus.)
Quests
Tortheldrin's death is required for the following quests: