The Ancient Helmet is obtained during the quest Time-Out when you free Theodore Caedmonessunu from his captors in the fort. The helmet is found in the area where Theodore can be found. It is worn by the last owner, until you take it from the corpse. This helmet is needed for the Last Legionnaire achievement it is one of the few items which a bonus only for the Thegn of Skjern.
This steel helmet looks like it has been heavily damaged, but skillfully repaired several times. Its design is ancient, but its ornamentation remains intact. It must have belonged to a man of high rank.