Speak with Lord Itharius again inside the Temple of Atal'Hakkar.
I must confess I have a more personal involvement in this affair. One of the first of the green dragons corrupted here was my brother, Eranikus.
I thought Eranikus gone, lost forever to the darkest rifts of nightmare, but I have heard that his shade has emerged within the temple. I know not whether it is truly him, or an empty, mournful echo.
If you wish to help me further, return to the Hall of Masks and journey through the archway into the true sanctum of the temple. I will find you there.