Before Heart of Stone: You can reclaim the mask from Xenia at the entrance to the Lumbridge Catacombs. During and after Heart of Stone: You can reclaim the mask from Ilona at the entrance to the Lumbridge Catacombs.
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 without any Defence level requirement.
The mask requires five parts to be made. The first four parts can be received by killing monsters within the Lumbridge Catacombs. The fifth part is always received by killing Dragith Nurn, who appears after touching the diamond demon statuette. Assembling the mask pieces completes the Mask of Dragith Nurn achievement which is a requirement for the master quest cape, and unlocks a cosmetic override of the mask.
It is impossible to obtain a second mask by obtaining a second set of five mask parts while the first set is still in the backpack. However, storing the first mask in the player-owned house will allow the player to obtain a second mask. Using a keepsake key does not allow the player to obtain more masks.
Although the destroy text claims that it can be reclaimed from Xenia/Ilona when destroyed, this is currently not possible.
Pieces of the Mask of Dragith can now be dropped from catacomb creatures if they were previously destroyed, allowing players that require the drops for the Master quest cape requirement to obtain them again.
If a player talks to Xenia while wearing the mask, she remarks on the player's ridiculous appearance: "Oh, take it off, [player]. It doesn't suit you. You look ridiculous!". Similarly when talking to Wizard Ilona during and after Heart of Stone she mentions Xenia hoped the player would put the mask back together following with the same remark.
Before the Evolution of Combat this mask granted a +1 to all offensive stats, being one of the few head slot items to do so and the only Free-to-Play item to offer such bonuses.