How do you obtain Duncan's Shield in Dragon Age Origin?

Answer 1

You can only obtain it near the end of the game. During the initial part of the Landsmeet mission chain, you will be asked to rescue Queen Anora from Arl Howe's estate. During this mission, you will meet and rescue Riordan, a Grey Warden from Orlais.

Back at Arl Eamon's estate, you can speak to Riordan who will thank you for the rescue and mention the location of a Grey Warden cache in Denerim. You may have already been there earlier, it is the warehouse location that contains a blocked off area behind a bookcase. When you visit it again, you will be able to move the bookcase and access the cache, which contains useful items including Duncan's Shield, a gift item that turns into a usable shield when gifted to Alistair.

Answer 2

To obtain Duncan's Shield in Dragon Age: Origins, you need to complete the quest "A Test of Faith" given by the Tranquil Proprietor in the Denerim Market District. This quest involves fighting your way through the Earl of Redcliffe's Estate to recover the shield. Once you defeat the revenant boss at the end and exit Denerim Market District, the shield will be in your inventory.