How do you get the 2015 Worlds: SKT T1?
It was sold in the League of Legends store. Riot's description in the client reads: “This icon was released in the store during the 2015 World Championship.”
Is the 2015 Worlds: SKT T1 still available?
Probably not. Riot tied the 2015 Worlds: SKT T1 to 2015 (store purchase), and it does not normally bring back past event rewards or bundles.
