Hail 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 3 waves of elementals, followed by Magnimaclus. If the event is not up, The Maelstrom will say "the storm has subsided."
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.