The Prison of Immol'thar is an area of Dire Maul West. It is located beyond the Court of the Highborne and leads on into the Athenaeum. The prison is sealed by a locked door that requires the Crescent Key to open.
The prison itself is a large building protected by a pulsating blue forcefield powered by five pylons. One pylon can be found in the Capital Gardens, two in the Court of the Highborne, and two just outside the prison. Each of the five pylons will be shut off when all of the elementals surrounding it have been dispatched. The pylon will flash when the final elemental is killed, indicating that it has been shut off.
When all five pylons have been shut off, the blue forcefield will fail, and the Highborne Summoners that were channeling the spell will be killed.
The area around the prison is rife with air elementals and mana surges who roam back and forth at high speeds. These can quickly ambush a party trying to cross the room, so it is wise to clear them out before attempting to shut down the pylons.