In Dragon Age Origins, The Arcane Warrior is a spellcaster with the added skill of being able to wear heavy armor. In order to become a Arcane Warrior, you must complete the following steps:
In order to unlock the Arcane Warrior class: During the Dalish Elves quest chain, you will be exploring a ruin in the middle of a forest. The Arcane Warrior specialization is taught by a spirit trapped in a gem in a room in the Lower Ruins. Make sure to explore each and every room and highlight all items (TAB on PC version). A conversation will be triggered upon touching the soul gem and it will ask you to release it from the torment it has endured by being imprisoned for centuries. Make sure to ask it to teach you the Arcane Warrior skills first before destroying the gem on the pedestal in the same room.
Congratulations! Now, your Mage characters can specialize in the Arcane Warrior class.