War Mask is one of the three skins released under the Ronin's Revenge part of the Samurai offer. It could be purchased as part of an item deal for a limited time in mid October 2017.
War Mask could be purchased for $4.99 in a deal that included 3250 coins. It could also be purchased for $19.99 in the "Best Deal!" which included the Skull Samurai 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 Ronin's Revenge sub-offer has been released multiple times after the initial 2017 debut. While the skins remained the same, the prices and included game items varied between releases. The details for all additional limited time releases for the War Mask skin are as follows: