WARNING: The lift to VhalSera does NOT go down. MAKE SURE ALL GROUP MEMBERS ARE ON THE LIFT BEFORE USING IT. If someone is not on the lift or in the room with the lift when it goes up, make sure they either teleport to you somehow or stay where they are. If they try to get to you and use a mirror in VhalSera's Dominion, it will teleport the whole group to that mirror and you will not be able to complete the zone.
WARNING #2 This quest is repeatable without any issues many times over until you defeat Tserinna at the very top of the tower. After you have defeated Tserinna you can still pick up the quest and collect the mirror shards, however the last step of placing the mirror shards in the frame (which would rise the platform to take you to VhalSera) bugs. The mirror in which you need to place the shards is unclickable.If there is someone who is in your group who has not defeated Tserinna before they can click the mirror however and update the quest for the group.
Simply picking up this quest (by standing on the first pedestal) can update the quest The Tower of Frozen Shadow for the relevant step. For this to work, you must not already have the quest "Mirror, Mirror" 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.