Shuriken is one of the three skins released under the Warriors of Steel part of the Ninjas vs Robots offer. It could be purchased as part of an item deal in early September 2017.
Shuriken could be purchased for $4.99 in a deal that included 3250 coins. It could also be purchased for $19.99 in a deal that included the Shadow 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 skin can vary depending on location, platform or time.
Note: The Warriors of Steel sub-offer was released again for a limited time in late April 2021. The item deals for Shuriken remained the same for this release. However, the browser exclusive deal with 50000 coins was replaced with 1800 DNA.