Oathkeeper

Oathkeeper is a steel longsword made by the Chantry to be used by Templars. It is received as a reward for completing all three of the Chanter's Board quests in Lothering.

History
A blessed blade, these longswords are often commissioned by the Chantry and placed in the hands of their most able templars as rewards for their accomplishments. (in-game description)

Trivia

 * This sword looks exactly like Duncan's.
 * Since George R. R. Martin's Song of Ice and Fire books are an inspiration, the name Oathkeeper may be an homage to the sword Jaime Lannister gave Brienne of Tarth in A Storm of Swords.