From Guild Wars Wiki
The Mask of the Mo Zing can be obtained from Vael inside Grothmar Wardowns (right outside of Longeye's Ledge) for 250 Superb Charr Carvings.
- Note that Vael will appear as a collector only after you have done all of his quests. If any person in your party hasn't finished them he will appear as a quest giver near the center of the map.
- Has armor rating and bonuses equivalent to normal headgear for the profession.
- Can be upgraded by runes and insignias.
- Can be dyed.
- If you happen to have an Eyepatch, it will not clip through.
- Gives <Item's Attribute + 1>

Bug.
The mask sticks to your face, so if your character has a beard, it will clip through the mask. (also with certain headdresses of female characters.)
[edit] Trivia
- According to Guild Wars lore, a Mo Zing is a masterless person. Zenmai is one such assassin.