Scythe is one of the three skins released under the Rider from Hell part of the Apocalypse offer. It could be purchased as part of an item deal for a limited time in mid December 2017.
Scythe 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 Warhorse skin and 1800 DNA. Scythe 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, platform or time.
Note: The Scythe skin was removed from future releases of the Rider from Hell sub-offer.