Kill a corrupted Elemental Lord. In any of the three caves at ( 49, -105, 244 ) /waypoint 48.96, -104.91, 244.12. Right click and turn three of the orbs to the same attunement (the alcoves will all match), then activate orbs by clicking orb to the left of each cave door (as you're facing into the room).
The quest Elemental Mystery requires an Elemental Lord of Poison that can only be spawned on the appropriate step of one of the three daily Cleaning the Elements quests.
The Void beasts in the Golden cave have an extremely short aggro range. You do not have to clear the room to activate the orbs, just stand within the center circle of the cave and you can Infuse/Activate all the orbs without fighting. The center MOB is non-aggro.