Skull Claus is one of the three skins released in the multi-part Mino's Christmas Special offer. It could be purchased as part of an item deal in late December 2018.
Skull Claus could be purchased for $9.99 in a limited time deal that included 25000 coins, 1200 DNA, Mistletoe Madness and Cuppy. 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: Skull Claus was released again for a limited time under the Skulls of Agario offer in mid August 2020. This time Skull Claus could be individually purchased for 5200 coins.