A'dal in Shattrath City wants you to slay Magtheridon.
Four trials you must pass before you are deemed worthy by the naaru.
Only one obstacle remains, mortal. Gather an army and travel to the lair of Magtheridon, deep inside Hellfire Citadel. Battle your way to the heart of the chamber and destroy the foul corrupter!
You will receive: 13 20
You have proven yourself as a being of great power. The naaru grant you access to Tempest Keep.
Upon completion of this quest you will gain:
Magtheridon's Lair is located in Hellfire Citadel; the instance portal itself is on the "ground level" in Hellfire Basin. It requires a 25-man raid to kill Magtheridon.
As of Patch 2.1.2, this quest and [The Tempest Key] quest reward are no longer required to enter The Eye. However, it will grant the title of Champion of the Naaru in Patch 2.3.0.
This is [The Tempest Key] quest chain. Prerequisite: The Cipher of Damnation quest chain
-  The Tempest Key
- [70R] Trial of the Naaru: Magtheridon