Dwarven Defender

Similar to the Dead Metal Bucket, and Nugbane the method of acquiring this crossbow is either bugged or it was simply never placed on an NPC.

Workaround
How to add the Dwarven Defender to your game:
 * 1) This requires that you have downloaded the toolset.
 * 2) Open the toolset, click the creatures tab. Scroll down until you see, "A Paragon of Her Kind". Open the folder, find orz200ar_commons.
 * 3) Open this folder, and scroll down to orz200cr_guardsman (Captain of the Guard). Right click this resource, and select "Open Local Copy". When the resource screen opens on the left, scroll down until you find inventory.
 * 4) Click to open and select Global from the folders on the left. Find Weapons, Ranged followed by Crossbows, Unique and finally Dwarven Defender. Add it to his inventory, make sure it is set to "Not Equipped" and then put a check in the "Stealable" box. Click Ok, and close the inventory screen.
 * 5) Export without dependent resources.
 * 6) Close the toolset (saving if prompted) and start up the game, load a save before Orzammar has been visited and travel there to steal your new prize.