Golden Mask
The Golden Mask is used to grant access to the Carnival. 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.
How to Obtain
Once Ezio completes the three carnival challanges and he defeats the Templar Soldiers and Dante Moro in a riged fight. He was supposed to recieve the Golden Mask as your reward, but Silvio Barbarigo payed off the Carnival announcer to give the mask to Dante.
Ezio,Antonio and Sister Teodora plan for Ezio to take the mask back. Ezio suggests killing Dante but Teodora says to steal the mask from him using her girls to distract him.
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.