The Brecilian Ruins are deep in the East Brecilian Forest. You go here during the Nature of the Beast quest to rid the Dalish Elves of their Werewolf plight. In the Ruins you will fight undead, Werewolves, and encounter more of the Black Vial phylacteries which unleash Revenants. You will also be able to unlock Arcane Warrior in the Lower Ruins.
 Ancient History
The Brecilian Ruins are actually an ancient Elven sanctuary of peace and tranquility, where the Eldest would go to slumber and where other Elven visitors go to pay tribute to their gods. The interior designs and rooms still retain some of the ancient beauty of the Elven people, as well as hiding some of their traditions and secrets.
There is some allusion to the ruins having originally been built by Humans, as remembered by the Spirit in the Phylactery. There's also speculation that both humans and elves lived there. This reinforces some of the lore learned in the Dalish origin whereupon the would be Warden discovers a cavern that appears to have been inhabited by both Elves and Humans in ancient times.