ARCANE WARRIOR
This specialization can be unlocked while doing the Nature of the Beast quest line. It is located in the Lower Ruins of the Brecilian Ruins. In the north-east corner of the map (Brecilian Forest Lower Ruins Map, L11), just south of the Elven Burial Chamber, the corridor forks east and west into dead ends. The room is in the dead end to the west.
Inside a small chamber which looks like a ruined library, there is a Broken Stone Altar. The phylactery is hidden in the far right corner of the room, beside the headless statue holding a shield. (It is not automatically highlighted, so for Xbox use the left trigger, for Playstation use L2 and for PC use TAB to highlight all the items in the room.)
When you touch the phylactery you experience the memories of an elven arcane warrior who has remained trapped inside of the phylactery for centuries. It offers to teach you the secrets of the Arcane Warriors in exchange for setting its spirit free by placing the phylactery on an ancient altar. Although the spirit has been imprisoned for so long that it has forgotten where the altar is located, the altar is directly to the left. By receiving the memories from the phylactery, you unlock the Arcane Warrior specialization for the mage as well as 250 XP.
In order to unlock the specialization, interact with the phylactery.
{Touch the gem.}
{Is this some kind of trapped spirit?}
{Is there anything I can do for you?}
{Yes. I will try to help you}
{Approach the stone altar with the gem.}
{Yes give me your memories}
{Place the artifact on the stone altar.} Alistair, Leliana (+2)
Nah. {Toss the artifact aside.}
Sorry, you may have a use later.