Vicious is one of the skins released under the Party part of the Agar.io Birthday offer. It was available for purchase for a limited time in mid July 2017.
Vicious could be purchased for $9.99 as part of the Birthday Pack. This pack also included the Angry skin with 7000 coins and a Mass Boost (3X, 24H) on browser or 1800 DNA and a Birthday Potion on mobile. Vicious 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: Vicious was released again for a limited time as part of the All the Internet Things offer in mid November 2019. This time Vicious could be purchased individually for 2600 coins.