By the Time You Hear This

After you finish the level 80 trial "The Dying Grasp" speak with the NPC Bethana at X:8.5 Y:10.9 in The Cystarium to start the quest.

She will then move to X:12.7 Y:10.6 to continue the quest and open the dungeon "The Twinning".

There was a quest marker that had popped up for me at X:13.7 Y:11.2 as if to continue the quest before I had cleared the dungeon but it is simply marking the location of the objective of the quest.