Basilisk is one of the six skins released under the Magical Creatures offer. It could be individually purchased using game coins in December 2016.
Basilisk could be purchased in the Skins Shop under the 'Magical Creatures' tab for 2900 coins. It was available for a limited time and may not be available for purchase in the future.
Details of this offer can vary depending on location, platform or time.
Note: The Magical Creatures offer has been released multiple times after the initial debut in 2016. Additional limited time releases were in late March 2018, mid October 2019 and then in early October 2020. The price for Basilisk remained at 2900 coins for each release.