Only one charm drops per kill, so if in a group, the summoning must be done by each person individually.
Go to The Temple of Cazic-Thule and kill Thulian Arch-Guardian. To summon the guardian, place the charms in the statues at the entrance, at ( 7, 7, 145 ) Copy/waypoint 7, 7, 145 and ( -7, 7, 145 ) Copy/waypoint -7, 7, 145. The guardian will spawn at the top of the nearby stairs and slowly walk down.
If you fail to kill him you must wait ~10 minutes before you can start the event again.