Deathly Mine is one of the three skins released in the multi-part Cap'n Joe's Shop offer. It could be purchased as part of an item deal in mid July 2018.
Deathly Mine could be purchased for $14.99 in a limited time deal that included 25000 coins, 1200 DNA and the Fake Chest skin. This skin 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: Deathly Mine was released again for a limited time under the Skulls of Agario offer in mid May 2020. This time Deathly Mine could be individually purchased for 4500 coins.