You must destroy the guardians surrounding the 5 Pylons that power the Prison of Immol'thar. Once the Pylons have powered down, the force field surrounding Immol'thar will have dissipated.
Enter the Prison of Immol'thar and eradicate the foul demon that stands at its heart. Finally, confront Prince Tortheldrin in Athenaeum.
Return to the Shen'dralar Ancient in the courtyard should you complete this quest.
This madness must come to an end. The age of the immortals has long since passed. Destroy the guardians surrounding the five pylons to lower the force field. When this is done, destroy Immol'thar. With the Prince's power greatly reduced from the demon's demise, attack and end him as well. Lay the spirits of this city to rest, adventurer, and make the Prince pay for his wrongdoings.