Golden Mask
The Golden Mask is used to grant access to the Carnevale. It can only be used once at the very end of a mission. Assuming an update will be out that enables you replay your core memories, you should be able to go back to the Memory Sequence. This will be the only way to wear the Golden Mask again. It does not offer the effect the normal Carnevale mask gives.
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 Teodora says to steal the mask from him using her 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.