Within several of the explorable areas, there are lanterns that can only be lit by a mage. I know there is one in the Western Approach but I have also seen another in Exalted Plains. These runes unlock crafting schematics.
The one is the Western Approach is the one that I completed. It can be found right after entering Coracavus as part of the Trouble With Darkspawn quest. Right after you enter, there will be an area that is a jail/torture chamber. The lantern is located in one of the cells and the rune you need to find is located in a room by the stairs.
That is everything that you need. Enjoy the new rune schematic!