When the Imperium occupied the area that is present-day Ferelden, they had two sites dedicated to magical experimentation at the extreme ends of the Imperial Highway. The southern one was the fortress of Ostagar, which looked out over the Korcari Wilds. The northern one was Aeonar, although the exact location is now a secret known only to a handful of Templars.
Whatever it was the Tevinter were trying to discover at Aeonar, their work was never completed. The fortress was overrun by disciples of Andraste upon hearing the news of her death. According to legend, it was a massacre—eerily silent, for the invaders caught the mages while all but one of them were in the the Fade.
The site was left structurally sound but spiritually damaged. Possibly because of this, the Chantry chose to put it to use as a prison. Accused maleficarum and apostates are held in the confines of Aeonar. Those who have a powerful connection to the Fade, and particularly to demons, will inevitably attract something across the Veil, making the guilty somewhat easier to tell from the innocent.