Thieve's Mask

"It's in Alerin City in a secret Cellar. You'll need a password to enter. It seems silly, but it's "shadow". It's not as simple as it sounds though"

- Puck

Mask is one of the possible rewards of the from the Tough Decisions Quest.

Wearing this as an identity to join a guild. This is not confirmed and could be one of the future feature that has yet to be released.