How do you get the Pengu (Ruby) Icon?
It came in a bundle. Riot's description in the client reads: “This icon was released in the store for purchase as part of a special bundle during Pool Party 2018 event.”
Is the Pengu (Ruby) Icon still available?
Probably not. Riot tied the Pengu (Ruby) Icon to 2018 (bundle), and it does not normally bring back past event rewards or bundles.
