How do you get the 2015 IWCI?
It was sold in the League of Legends store. Riot's description in the client reads: “This icon was released in the store for purchase during the International Wildcard Invitational 2015.”
Is the 2015 IWCI still available?
Probably not. Riot tied the 2015 IWCI to 2015 (store purchase), and it does not normally bring back past event rewards or bundles.
