Seal Knight is one of the three skins released under the Morgana's Castle offer. It could be individually purchased using game coins in March 2017.
Seal Knight could be purchased in the Skins Shop under the ‘Excalibur’ tab for 1990 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 Morgana's Castle offer was released again for a limited time in mid June 2018 and then in mid August 2019. The price for Seal Knight remained the same for each release.