Vicious is one of the skins released under the Party part of the Agar.io Birthday offer. It was available for purchase 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 and platform.