I submit this theory: Hawke is the one to kill the Nightmare.
We all know the choice reads “will likely die” to cover the Inquisitor’s escape. I suspect, no matter how heart-breaking Varric’s reaction is, that leaving Hawke behind is ultimately the right decision.
First: “The world fears the inevitable plummet into the abyss. Watch for that moment… and when it comes, do not hesitate to leap. It is only when you fall that you learn whether you can fly” that has already been pointed out before in relation to the quest, actually bloody called Here Lies the Abyss. It reads like a narrative connection, a clue—foreshadowing, if you will. Nothing is ever titled without a very specific purpose in mind.
Second, Hawke is the only person we have seen so far that has the ability to overcome the very essence of Nightmare: ancient, primal, debilitating.
Beyond any shadow of doubt, the Nightmare’s taunt for Hawke is the most gut-wrenching: “Did you think anything you ever did mattered?”
Because we all know, it did not.
This thing that frustrated us as gamers—the idea that no matter what decisions we made in an RPG title that usually lets us control the fate of nations, nothing changed—becomes the catalyst of Hawke’s character, the well from which they draw strength. Again and again, Hawke faces personal waking nightmares: from the Darkspawn attack on their home that claims one of the twins’ lives, to ultimately losing their other sibling (either to the Taint/Wardens, the Gallows, or the Templars), to the horrific death of their mother no matter how hard they fight, no matter what decision they make in the investigation of her murderer.
And then, Hawke has to face the nightmares of nations. An invisible war: the faithful spreading propaganda and crucifying heretics. A Qunari invasion of the city. An underground rebellion. Growing tensions between Mages and Templars that ultimately erupt into the start of a war. And then—personal again, even after all the loss: the lies of a lover, the unraveling of a friend, anger, betrayal, death—things explode around you, Hawke. And there’s not a damn thing you can do to prevent it.
All while remaining a pillar of leadership for the friends who have become family.
When there’s nothing left to do but survive, to hold those you love as close as you possibly can, you grow strong, knowing you’ve faced your worst nightmares again and again and you’re still standing.
This is the strength of Hawke.
It is the strength that will overcome the Nightmare, as it has overcome so many (so impossibly many; so unfairly many) before.