Before the quest The Shadowed Sun, make sure you've gotten Mythal's Essence so she can help you reason with Solas. You should also have all factions at maximum strength and all companions become a ...

So, make sure you have lots of saves on hand so you can jump back if things do go awry. This missable trophy called 'Clear Minds and Open Hearts' involves Mythal, a big player in why Solas is the way ...