Kill a corrupted Elemental Lord. In any of the three caves at (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.
The quest Elemental Mystery requires an Elemental Lord of Posion that can only be spawned on the appropriate step of one of the three daily Cleaning the Elements quests. If you are attempting to complete this timeline in short order, do not summon the Elemental Lord a 2nd time, until you are also on the quest Elemental Mastery. Doing otherwise, will incur a 24hr lockout before completing that part of the timeline. Likewise, do the next Cleansing, after Elemental Mastery, for the 2nd needed Elemental Lord