The Cipher of Damnation is indeed a powerful incantation. Ancient and chaotic, the cipher itself has been responsible for many tragedies in the history of our worlds. That is to say, it is not unique to Draenor.
The burning symbol you saw was the mark of Kael'Thas. His knowledge of the ancient words explains much.
Your discovery has roused the naaru. A'dal has requested your presence in Shattrath City. Should you choose to come, seek me out at the Terrace of Light.
This letter is meant to direct you back to Khadgar and A'dal. A'dal will start you on the quest chain to get The Tempest Key which opens The Eye wing of the Tempest Keep. See the Tempest Key guide for more information.