Click on the sword on the altar, The Maelstrom. If the event is ready it will ask you if you are prepared to face the storm. If you say yes, this will trigger three waves of elementals, followed by Magnimaclus. If the event has been completed recently, The Maelstrom will say "The power of the storm has subsided."
Once you have started the ring event, the door will lock. After you have defeated Magnimaclus, click on the sword again to get the message "The power of the storm has subsided" and the door can be opened again.
Known Bug: The mobs associated with the ring event can sometimes spawn beneath the ground, above the ceiling, or outside the walls, putting them out of range of attacks (even AoE). If this happens then you're SoL and must wait until the event can be started again. The event respawn is ~40 real-world minutes.
