Blessed Medallion of Karabor

The Blessed Medallion of Karabor is an upgraded version of the original, which allows entry into the Black Temple. Its further blessing allows the wielder to teleport directly to the Black Temple's gates in order to quickly continue the battle against Illidan's forces.

Source
The Blessed Medallion of Karabor is obtained after completing The Fall of the Betrayer, which involves killing Illidan Stormrage in the Black Temple.