Demon is one of the skins released in the monthly Titans Skin Collection. It could be purchased as part of an item deal in early January 2020.
Demon could be purchased for $4.99 in a limited time deal which included a package of 5000 coins. 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 Titans Skin Collection with the Demon skin was released again in late May 2021. The price and included coins for Demon remained the same for this limited time release.