Supremus is one of the three skins released under the Ancient Knight part of the Mechanos offer. It was available for purchase for a limited time in late June 2017.
Supremus 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 Mechatron skin and either 50000 coins (Browser) or 1800 DNA (Mobile). It 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: The Ancient Knight sub-offer with the Supremus skin was released again for a limited time in early February 2019. This time the deal that included 3250 coins was lowered to $1.99. The Best Deal was lowered to $9.99 with 1800 DNA being the only included item package.