It is given by Owen as a reward for the effort of the Warden to find his daughter, Valena. He claims that this trinket belonged to his master from a long time ago and it is better in the Warden's hands than his.
In order to acquire this item, you should activate the quest Lost in the Castle and not find Valena during the events in the Redcliffe Castle. When you return to Owen he will ask you what happened. Choose the option "Yes, I suppose it means she must be dead." Owen will thank you and reward you with this item. Choosing any other dialogue option won't grant you this helmet.
- Apparently this light helmet's material is Grey Iron, which is metal. It is unclear why this is the only light helmet made of metal and not leather, but as it is described as 're-forged,' the material it was chiefly made of can be assumed.