A knight's helmet is an item in Evony TKR (The King's Return).
Knight's Helmet is one of the collection items in the exhibition for the Art Hall. It is one of four parts in the Knight Equipment Hall. After the set is completed at three stars, you can trade additional Knight's Helmet fragments for gold.
The description of Knight's Helmet in Evony TKR says:
Equipment worn by the knights. The important exhibits in Knight Equipment Hall.
Knight's Helmet is known to potentially be obtainable from opening Artwork Fragment Chest V and Artwork Fragment Chest VI. The links for each item are below:
Note: This list may be incomplete.