I think in Dragon Age 2 Merrill or Fenris (I really don’t remember, and maybe it was in a DLC, but I think it was one of these two) mentions at some point that a big Arlathvhen is going to happen soon-ish somewhere in the Dales (I could be mistaken though).
Anyway though: if there was one relatively shortly before the events of Inquisition (maybe particularly the clans coming together to discuss the upcoming Conclave and the Mage-Templar conflict, maybe already planning to send at least one of their own to spy on the event), it could mean that Lavellan most likely was in the Dales before. And even if not I’ll just adapt it as a headcanon because I think it would be at least somewhat logical. And it would also fit with the Arlathvhen being hold about every 10 years. I think there was one shortly before Origins (so probably 9:28 or 9:29) and this one would then maybe take place inbetween 9:38 and 9:40, roughly 10 years after the last one.
Now I had to imagine Tamlen (my Tamlen Lavellan, just for clarity’s sake) returning to the Dales with the Inquisition later. Maybe the Arlathvhen took place in the Emerald Graves or the Exalted Plains, because both places are so important and significant in Dalish history. Meeting in the place where they once were defeated and had their final stand against the humans, to now again come together because an important event is about to happen that might change Thedas future forever. This meeting is before the Civil War in Orlais starts, so while the Dales of course are under human control they are still in a more or less peaceful state. It would pain Tamlen to see the Andraste statues and all the reminders of the Dalish elves’ biggest defeat, the human settlements, sure. But I can only imagine to how it must have felt to return only a few years later to the same place in the Exalted Plains, the remaining trees burned down, the smell of death and decay in the air, human soldiers everywhere fighting each other and destroying yet another piece of land that once was given to and belonged to his ancestors. Yet another instance of humans taking whatever they want and then destroying it and whatever history it might bear - because who cares, since it’s “only” elven history. I think a journey to the Dales is always bittersweet and possibly also painful for a Dalish, seeing what they lost in a stupid conflict so long ago and still couldn’t reclaim, being reminded that they still couldn’t find a new home for their people, and that they’re still not welcome in their own lands. But then additionally seeing everything destroyed or under so much human influence and affected by the Civil War as the Dales are during the time of Inquisition… it must be heartbreaking to see.
At first Tamlen would just be shocked to see the Exalted Plains’ current state, but very soon the first shock would turn into anger, the anger of thousands years of oppression that just doesn’t seem to come to an end.