Excalibur is one of the three skins released in the Excalibur - The Sword of Fate offer. Excalibur is also the legendary sword wielded by King Arthur. It could be purchased as part of an item deal in March 2017.
Excalibur could be purchased for $9.99 in a limited time deal that included 16000 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 Excalibur offer with the Excalibur skin was released again for a limited time in early January 2019. The price and included coins remained the same for the Excalibur skin.