Cthulhu is one of the skins released in the monthly Titans Skin Collection. It could be purchased as part of an item deal in early December 2019.
Cthulhu could be purchased for $4.99 in a limited time deal which included three 1 Hour Mass Boosts (2x) and three 1 Hour XP Boosts (2x). Cthulhu 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 Cthulhu skin was released again for a limited time in mid May 2021. The price and included items for Cthulhu remained the same for this release.