In the Haunt of Syl'Tor on the 7th Floor ("The Palindrome"), after defeating the unmasked changeling at around ( 36, 633, 10 ) /waypoint 36, 633, 10 and being teleported to around ( 36, 670, 10 ) /waypoint 36, 670, 10, and after moving south out of that room, the quest is automatically offered.
Simply picking up this quest (after the floor flips upside down) can update the quest The Tower of Frozen Shadow for the relevant step. For this to work, you must not already have the quest "Puzzled" in your journal, this quest must not already have been completed by anyone in this zone instance, and of course you must be on this step of the The Tower of Frozen Shadow quest. Note that the update is not obtained if you accept the quest being shared to you.
This quest is repeatable (although it is not marked in the journal as such), but will only give XP (and AA XP) once.
At the current time (30 Aug 2015), although after completion this quest is added to the "Tower of Frozen Shadow" section in the Completed Quests section of the Quest Journal, after subsequently defeating Tserrina and zoning out, the quest is then removed from the journal (so is not listed amongst the completed quests). So you may have completed this and not have it listed.