Edit: Ok, i think i figured it out. While i had no luck at fixing the bug, i looked up the scene on youtube and found out that i had actually already had the conversation with merril. For some reason, it will automatically have the final cutscene play if you talk to merril at the beginning of act 3.
I appoligize in advance for any possible spoilers
First off, upon attempting to start this quest, the cutscene at her house was bugged, in that it showed hawke and merril standing in the doorway, with the first response option available. i chose an option and it seemed to rectify itself. This is not the real bug, but thought i should mention it.
The real bug i was experiencing was after killing the pride demon possessed keeper and being confronted by her clan (as a note, i chose the option that caused me to kill off the clan). I then left the sundermount and the quest was marked completed. My problem is, i never saw the cutscene in which merril destroys the eluvian. It just marks the quest as complete and that is it. I also noticed that during all of act three, the mirror has been missing from her home, as if already destroyed.
Is anyone else experiencing this?
Here are some details on my play-through: -I have romanced merril, and she moved in with me -Merril has maxed out friendship -I have given her both gifts -I had sex with isabella, but ended up with merril afterwards -In isabella's final quest, i said i'd go on the ship with her and want merril to come with -I prayed at the shrine before going to the pride demon -I do have a good deal of mods, though none which affect merril (other than appearance)