The Mask of Dragith Nurn is an item obtainable by collecting five mask parts by killing monsters within the Lumbridge Catacombs. It has equal bonuses to an iron full helm but can be worn with any Defence level.
It is impossible to obtain a second mask by obtaining a second set of five mask parts while the first set is still in your inventory. However, storing the first mask in your Player-owned house will allow you to obtain a second mask. Using a keepsake key does not allow the player to obtain more masks.