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There is'nt a sword, great or long, that is named the "Dragon Sword." There is, however, a material in the game used to make weapons and armor called Dragonbone.
There is a sword called Yusaris however, which has a damage bonus vs dragons. This is obtained doing the summoning rituals in the circle of mages. A guide can be found here: http://www.dragonageguide.com/walkthrough/the-circle-of-magi/