There is no door key requirement to enter the Hero Magister Platform, but you need to complete a series of prerequisite tasks, and the first step of the prerequisite tasks is in Shattrath. Players from the Aldo camp must talk to the Guardian of the Light Adein in the center of the Aldo Highlands to receive the quest [Crisis at the Sunwell]. Players in the Astrologer camp must talk to Dathrai Sunshooter at the entrance of the Astrologer's Terrace Inn to receive the quest [Call of Duty].
For subsequent tasks, you need to clear the Magister Platform with normal difficulty. First, talk to the blood elf in the corner of the entrance to the No. 2 BOSS room to receive the follow-up task [Astrological Ball]. Then interact with the orb outside the exit of the No. 2 BOSS room and talk to Kalecgos who flew down. The mission [Survive the Catastrophe], after killing Kael'thas at the end, you must pick up the mission items to be considered complete.