Find Tserrina's veil in the chapel at ( 20, 355, 24 ) /waypoint 20, 355, 24.
Find Mayong's letter in the first antechamber at ( 52, 360, -148 ) /waypoint 52, 360, -148.
Find Tserrina's wine goblet in the second antechamber at ( -62, 363, -117 ) /waypoint -62, 363, -117.
Return to the chapel and observe the events. To trigger the event that you need to observe, you have to defeat many of the creatures in the chapel. A more detailed summary of the fights against the various creatures is given here.
Defeat the four golems in the chapel.
Defeat the a residual jester at ( -96, 356, 1 ) /waypoint -96, 356, 1 (he is stood amongst the child ghosts).
Defeat the a residual conductor at ( 60, 352, -32 ) /waypoint 60, 352, -32 (he is stood infront of the ghostly choir).
Defeat Ach'erin and his three accompanying zombies.
Defeat the encounter with High Priest Mohnt and Dizzina the Lulled (she slowly ambles down the aisle to join the fight when you engage the high priest). While fighting Dizinna, you can Flourish with the choir at ( 61, 352, -32 ) /waypoint 60.76, 351.67, -31.93 Type /Flourish to conduct the choir and stop her from regenerating her health. Near the end of the fight, you'll need to move to the children and /dance. This will decrease her healing again.
When that encounter is defeated, non-attackable versions of Tserrina and Mayong will appear and talk. Watch this conversation.
Simply picking up this quest (by clicking on the gift box) can update the quest The Tower of Frozen Shadow for the relevant step. For this to work, you must not already have the quest "Not Him Again" 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.
As of 14feb15, have completed killing Tserrina via first part of Freeing the Dain quest, and upon returning to A Bride's Scorn level 5, and repeating Not Him Again, I find that the Veil is click-able by all in group. Three of the four in group have killed Tserrina once already.
Warning! This quest is repeatable without any issues many times over until you defeat Tserinna at the very top of the tower (Not the manifestation of Tserinna's Rage spawned in this instance). After you have defeated Tserinna you can still pick up the quest, however the quest them bugs because you will not be able to see the veil to click on it for the first update. Sadly, even if you have members in your group who have not killed Tserinna and can see the veil, it is an individual update and not a group one. Therefore even though you can pick up the quest again after defeating Tserrina, you will not be able to complete it again.