Story related, cannot be missed.
This trophy will unlock after you complete the mission "In Your Heart Shall Burn". See below for more information.
*** Spoiler - click to reveal ***So just as a heads up, once you start this mission say goodbye to Haven as an area to explore. If you have any remaining quests here it is recommended you finish them before starting this story mission. It's also recommended you be at least Level 8 to tackle this one.
So head to the War Room to begin this quest. Surprisingly, you're going to head out and seal the Breach. Wow, short game huh? No worries, just watch the cutscene (there are a lot in this mission) and after the Breach is closed you'll head back to Haven. Chat with Varric (what you say to him doesn't really matter outside of gaining or losing favor with him) and then set in for another cutscene.
After this scene you're finally back in control of your inquisitor as all hell is breaking loose. If you need to resupply there is a cache right behind you. Once you head to the docks Dorian will greet you and inform you that the Templars are attacking (I sided with the Mages in game, if you side with the Templars then Mages will now be arriving to fight you).
After this scene you now engage in some pretty straightforward battles as you make your way to the Trebuchet. At the first Trebuchet you'll have to deal with some mobs, be wary of the mages / archers that like you try and flank you. Once you defend this first Trebuchet, a second one will need your assistance. Head there now, mopping up enemies and looting when necessary. You'll need to manually fire this Trebuchet while all the while having mobs come at you. This is where making good use of the tactical camera comes in handy so you can have one person holding X while the others defend you. Once the Trebuchet is fired, you'll return to the front gates.
At this point you're greeted with an optional side quest of helping the people of Haven while your main mission is to regroup at The Chantry. I'm not sure how many people need help in all but rescuing them nets you 80 influence each. Be forewarned that some people are timed rescues and once you stumble upon them you have very little time to save them before they die.
Whether or not you choose to save people, to move the story along head to the Chantry. From here another custscene will take over. After it, you will need to head BACK down to the Trebuchet for one final shot. Deal with the mobs as you see fit and know that once you get to the Trebuchet there will be a supply cache (but it's only a single use so heads up there).
Once you begin winding the Trebuchet (by holding X in front of it) you will start to be attacked by waves on enemies. Tactical camera time! Defeat 3 waves, and on the fourth and final wave the leader of whichever faction you didn't side with will appear and destroy you (so empty your heads). Jokes aside you can fight this boss but aside from XP you're not going to get any good loot. However once this Wave is clear you are free to turn the Trebuchet at your leisure.
This next cutscene will finally introduce you to the big bad behind the game The Elder One. After this cutscene plays out you'll be down in an icy cave under Haven. Approach the Wraiths and you'll unlock a new AoE ability which is great to have. You'll also be fighting on your own at this point as your party members are not with you. There is little if anything to loot in the area so just keep heading forward. You'll go through several sequences of cutscenes then in game travel, just keep heading forward and you'll be alright. Once you finally catch up to your companions, you can move the story along more. Finally after a LOT of story (mind you I was invested as hell to this game so I loved it) you will finally arrive in Skyhold and can take up your mantle as Inquisitor. Once you do, the mission will end and you'll be the proud owner of this shiny new bronze trophy.