Trickster is one of the three skins released in the Carnival of Venice part of the Carnival offer. It could be purchased as part of an item deal for a limited time in mid February 2017.
Golden Mask could be purchased for $19.99 in the limited time "Best Deal!" which included the Trickster skin and either 50000 coins (Browser) or 1800 DNA (Mobile). It was not available for individual purchase and may not be available for purchase in the future.
Details of this offer may vary depending on location and platform.
Note: The Trickster skin has been released multiple times in separate offers after the initial 2017 debut. The details for all additional limited time releases for the Trickster skin are as follows: