Golden Mask
The Golden Mask is used to grant access to Marco Barbarigo's exclusive party during Carnevale. It is a storyline item that must be acquired in order to progress to the target assassination.
How to Obtain
Once Ezio completes the three carnival challenges and defeats the Templar Soldiers and Dante Moro in a rigged fight, he was supposed to receive the Golden Mask as the reward, but Silvio Barbarigo bribed the Carnival announcer to give the mask to Dante instead.
Ezio, Antonio and Sister Teodora plan on how to take the mask back. Ezio suggests killing Dante, but Antonio says to steal the mask from him using Sister Teodora's girls to distract him as killing Dante will surely cancel the party altogether.
Once you steal the mask, it goes into effect once you enter Marco Barbarigo's party, he must become hidden from the Templar guards under Dante while they look for the mask.
The Golden Mask is only used during this mission.