Razor Dragon is one of the three skins released under the Royal Pack part of the House of Dragons offer. It could be purchased as part of an item deal for a limited time in late September 2017.
Razor Dragon 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 Viper Dragon 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 may vary depending on location, platform or time.
Note: The Royal Pack sub-offer was released again for a limited time in late May 2019. This time the price for Razor Dragon and the 3250 coins was lowered to $1.99. The Best Deal was lowered to $9.99 and the included coins were reduced to 30000.