Cursed Blade is one of the three skins released under the Eye of the Void part of the Gladiators offer. It could be purchased as part of an item deal for a limited time in mid July 2017.
Cursed Blade 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 Orc Warrior 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 Eye of the Void sub-offer with Cursed Blade skin was released again for a limited time in late March 2019. This time the deal that included 3250 coins was lowered to $1.99. The Best Deal was lowered to $9.99 and included 30000 coins. The package of DNA was not an option on either platform.