Head to Silvermoon City or to Ironforge. Go to the flightmaster and fly to Quel’Danas Isle (Sunwell). You will see a few tents in front of the Boat. Talk to Exarch Larethor. He will give you this quest: Magister’s Terrace.
To find the Magister’s Terrace turn around to face the boat. Head east following the coast. Northeast you will see a road toward a hill. The Magister’s Terrace dungeon is up the hill. To see the type of mobs and their abilities in advance visit Blizzplanet’s Magister’s Terrace section along with Boss encounter videos.
This quest is important because once you find the NPC corpse by Vexallus’ room (Halls of Theory), the follow up is to click the magic ball at The Solar Vigil. Click the ball to run the in-game cinematic. This will unlock the Magister’s Terrace Heroic-mode and will trigger the spawning of Kalecgos who flies down to give you the follow up quest which requests you to bring Kael’thas’ head to Exarch Larethor.